feat: expose Lance metrics via OpenTelemetry in Python and Node (#3609)

Bridges Lance's internal `metrics`-crate instrumentation (object store
request counts, bytes, latency, errors, and throttles) into
OpenTelemetry, in both the Python and Node bindings, with a shared
adapter in the Rust core. This is the LanceDB counterpart to
lance-format/lance#7537.

## Rust core (`rust/lancedb`)
Two new, **off-by-default** features:
- `metrics` — re-exports the [`metrics`](https://docs.rs/metrics) crate
as `lancedb::metrics` and turns on Lance's object-store instrumentation.
Install any `metrics`-compatible recorder to collect them.
- `metrics-otel` — adds `lancedb::metrics_otel`, a pull-based adapter
that installs a process-global recorder aggregating into lock-free
cumulative storage and exposes a snapshot/catalog API
(`register_metrics_recorder`, `metrics_catalog`, `snapshot_metrics`,
`MetricPoint`/`MetricValue`/`MetricKind`/`MetricDescription`). Both
bindings build on this.

## Python
`lancedb.otel.instrument_lancedb_metrics()` registers each metric as an
OpenTelemetry observable instrument on the given (or global)
`MeterProvider`. Available via the `otel` extra (`pip install
lancedb[otel]`), which pulls in only `opentelemetry-api` — the
application supplies and configures the SDK.

## Node
`instrumentLanceDbMetrics()` provides the equivalent wiring against
`@opentelemetry/api`. This is the only public entry point; the
underlying recorder/catalog/snapshot functions stay internal.

Because OpenTelemetry has no asynchronous histogram instrument,
histograms are exported Prometheus-style as `<name>_bucket` (with an
`le` attribute), `<name>_count`, and `<name>_sum`. Only `_sum` carries
the histogram's unit; `_bucket` and `_count` observe cumulative counts
and are unitless. The adapter is enabled by default in the Python and
Node builds, and off by default in the Rust crate.

## Notes
- Requires Lance ≥ `v9.0.0-beta.19`, which ships the object-store
metrics APIs (upstream lance-format/lance#7537, now merged). `main` is
already on beta.19, so this is a single feature commit with no
dependency bump.
- Tests: 8 Rust unit tests, 3 Python tests, 2 Node tests, all covering
the end-to-end object-store-metrics → OpenTelemetry path.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

python/python/lancedb/_lancedb.pyi

@@ -30,6 +30,25 @@ IvfHnswPq: type[HnswPq] = HnswPq
 IvfHnswSq: type[HnswSq] = HnswSq
 IvfHnswFlat: type[HnswFlat] = HnswFlat
 
+class MetricPoint:
+    name: str
+    kind: str
+    attributes: Dict[str, str]
+    value: Optional[float]
+    buckets: Optional[List[Tuple[str, int]]]
+    count: Optional[int]
+    sum: Optional[float]
+
+class MetricDescription:
+    name: str
+    kind: str
+    unit: Optional[str]
+    description: str
+
+def register_lancedb_metrics_recorder() -> bool: ...
+def lancedb_metrics_catalog() -> List[MetricDescription]: ...
+def snapshot_lancedb_metrics() -> List[MetricPoint]: ...
+
 class PyExpr:
     """A type-safe DataFusion expression node (Rust-side handle)."""
 

python/python/lancedb/otel.py

@@ -0,0 +1,170 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+"""Bridge LanceDB's internal metrics into OpenTelemetry.
+
+LanceDB (through Lance core) publishes metrics (currently object store request
+counts, bytes, latency, errors, and throttles) through the Rust ``metrics``
+facade. This module installs a process-global recorder that aggregates them and
+registers OpenTelemetry observable instruments that report the aggregated values
+into the user's ``MeterProvider``.
+
+The bridge is generic: every metric LanceDB describes is surfaced automatically,
+with no per-metric Python code. Histograms have no asynchronous OpenTelemetry
+instrument, so each is exported Prometheus-style as cumulative ``le`` buckets
+plus ``_count`` and ``_sum`` observable counters.
+"""
+
+from __future__ import annotations
+
+import warnings
+from typing import TYPE_CHECKING, Optional
+
+from ._lancedb import (
+    lancedb_metrics_catalog,
+    register_lancedb_metrics_recorder,
+    snapshot_lancedb_metrics,
+)
+
+if TYPE_CHECKING:
+    from opentelemetry.metrics import MeterProvider
+
+_INSTRUMENTED = False
+
+
+def instrument_lancedb_metrics(
+    meter_provider: Optional["MeterProvider"] = None,
+) -> bool:
+    """Register LanceDB metrics as OpenTelemetry observable instruments.
+
+    Installs a process-global metrics recorder and creates one observable
+    instrument per LanceDB metric on the given (or global) ``MeterProvider``. The
+    user's configured ``MetricReader`` then collects them on its own schedule.
+
+    Counters and gauges map directly to observable counters/gauges. Each
+    histogram is exported as cumulative ``le`` bucket counts (``<name>_bucket``,
+    with an ``le`` attribute) plus ``<name>_count`` and ``<name>_sum``.
+
+    Parameters
+    ----------
+    meter_provider : opentelemetry.metrics.MeterProvider, optional
+        The provider to register instruments on. Defaults to the global provider
+        from ``opentelemetry.metrics.get_meter_provider()``.
+
+    Returns
+    -------
+    bool
+        ``True`` if the recorder is installed and instruments are registered.
+        ``False`` if a different ``metrics`` recorder is already installed in
+        this process (``metrics`` permits only one global recorder), in which
+        case a warning is emitted and no instruments are created.
+
+    Notes
+    -----
+    Requires the OpenTelemetry API (``pip install lancedb[otel]``) and, to
+    actually export, an OpenTelemetry SDK (``pip install opentelemetry-sdk``)
+    configured by the application. Calling this more than once is safe;
+    instruments are created only on the first successful call.
+    """
+    global _INSTRUMENTED
+
+    try:
+        from opentelemetry.metrics import Observation, get_meter_provider
+    except ImportError as exc:
+        raise ImportError(
+            "instrument_lancedb_metrics requires the OpenTelemetry API/SDK. "
+            "Install it with `pip install lancedb[otel]` or "
+            "`pip install opentelemetry-sdk`."
+        ) from exc
+
+    if not register_lancedb_metrics_recorder():
+        warnings.warn(
+            "Could not install the LanceDB metrics recorder: another `metrics` "
+            "recorder is already installed in this process. LanceDB metrics will "
+            "not be exported via OpenTelemetry.",
+            stacklevel=2,
+        )
+        return False
+
+    if _INSTRUMENTED:
+        return True
+
+    provider = meter_provider or get_meter_provider()
+    meter = provider.get_meter("lancedb")
+
+    def scalar_callback(metric_name: str):
+        def callback(_options):
+            return [
+                Observation(point.value, point.attributes)
+                for point in snapshot_lancedb_metrics()
+                if point.name == metric_name and point.value is not None
+            ]
+
+        return callback
+
+    def bucket_callback(metric_name: str):
+        def callback(_options):
+            observations = []
+            for point in snapshot_lancedb_metrics():
+                if point.name != metric_name or point.buckets is None:
+                    continue
+                for le, cumulative in point.buckets:
+                    attributes = dict(point.attributes)
+                    attributes["le"] = le
+                    observations.append(Observation(cumulative, attributes))
+            return observations
+
+        return callback
+
+    def field_callback(metric_name: str, field: str):
+        def callback(_options):
+            observations = []
+            for point in snapshot_lancedb_metrics():
+                if point.name != metric_name:
+                    continue
+                value = getattr(point, field)
+                if value is not None:
+                    observations.append(Observation(value, point.attributes))
+            return observations
+
+        return callback
+
+    for desc in lancedb_metrics_catalog():
+        unit = desc.unit or ""
+        if desc.kind == "counter":
+            meter.create_observable_counter(
+                desc.name,
+                callbacks=[scalar_callback(desc.name)],
+                unit=unit,
+                description=desc.description,
+            )
+        elif desc.kind == "gauge":
+            meter.create_observable_gauge(
+                desc.name,
+                callbacks=[scalar_callback(desc.name)],
+                unit=unit,
+                description=desc.description,
+            )
+        elif desc.kind == "histogram":
+            # `_bucket` and `_count` observe cumulative sample counts, not the
+            # histogram's measured quantity, so they are unitless; only `_sum`
+            # carries the histogram's unit.
+            meter.create_observable_counter(
+                f"{desc.name}_bucket",
+                callbacks=[bucket_callback(desc.name)],
+                description=f"{desc.description} (cumulative buckets)",
+            )
+            meter.create_observable_counter(
+                f"{desc.name}_count",
+                callbacks=[field_callback(desc.name, "count")],
+                description=f"{desc.description} (count)",
+            )
+            meter.create_observable_counter(
+                f"{desc.name}_sum",
+                callbacks=[field_callback(desc.name, "sum")],
+                unit=unit,
+                description=f"{desc.description} (sum)",
+            )
+
+    _INSTRUMENTED = True
+    return True