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>

nodejs/src/lib.rs

@@ -12,6 +12,7 @@ mod header;
 mod index;
 mod iterator;
 pub mod merge;
+pub mod otel;
 pub mod permutation;
 mod query;
 pub mod remote;

nodejs/src/otel.rs

@@ -0,0 +1,119 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+//! Node.js bindings over [`lancedb::metrics_otel`].
+//!
+//! The aggregation, catalog, and histogram bucketing all live in the LanceDB
+//! core crate; this module only converts the core snapshot types into napi
+//! objects and exposes the three entry points to JavaScript, where
+//! `lancedb/otel.ts` bridges them into the user's OpenTelemetry `MeterProvider`.
+
+use std::collections::HashMap;
+
+use lancedb::metrics_otel::{MetricPoint as CoreMetricPoint, MetricValue};
+use napi_derive::napi;
+
+/// One cumulative histogram bucket: all samples with value `<= le`.
+#[napi(object)]
+pub struct MetricBucket {
+    /// The inclusive upper bound of the bucket, or `"+Inf"` for the final bucket.
+    pub le: String,
+    /// Cumulative number of samples less than or equal to `le`.
+    pub cumulative_count: f64,
+}
+
+/// One aggregated metric data point. For counters and gauges only `value` is
+/// set; for histograms `buckets` (cumulative `le` counts), `count`, and `sum`
+/// are set.
+#[napi(object)]
+pub struct MetricPoint {
+    pub name: String,
+    pub kind: String,
+    pub attributes: HashMap<String, String>,
+    pub value: Option<f64>,
+    pub buckets: Option<Vec<MetricBucket>>,
+    pub count: Option<f64>,
+    pub sum: Option<f64>,
+}
+
+impl From<CoreMetricPoint> for MetricPoint {
+    fn from(point: CoreMetricPoint) -> Self {
+        let kind = point.kind.as_str().to_string();
+        let (value, buckets, count, sum) = match point.value {
+            MetricValue::Scalar(v) => (Some(v), None, None, None),
+            MetricValue::Histogram {
+                buckets,
+                count,
+                sum,
+            } => (
+                None,
+                Some(
+                    buckets
+                        .into_iter()
+                        // Counts stay well within the f64-exact integer range
+                        // (2^53), so this cast is lossless in practice and keeps
+                        // the values plain JS numbers for OpenTelemetry.
+                        .map(|(le, cumulative_count)| MetricBucket {
+                            le,
+                            cumulative_count: cumulative_count as f64,
+                        })
+                        .collect(),
+                ),
+                Some(count as f64),
+                Some(sum),
+            ),
+        };
+        Self {
+            name: point.name,
+            kind,
+            attributes: point.attributes,
+            value,
+            buckets,
+            count,
+            sum,
+        }
+    }
+}
+
+/// A described metric, used by the JavaScript layer to create instruments up front.
+#[napi(object)]
+pub struct MetricDescription {
+    pub name: String,
+    pub kind: String,
+    pub unit: Option<String>,
+    pub description: String,
+}
+
+/// Install the LanceDB metrics recorder as the process-global `metrics` recorder.
+///
+/// Returns `true` if the recorder is installed (now or previously). Returns
+/// `false` if a *different* recorder is already installed — `metrics` allows
+/// only one global recorder per process, so LanceDB cannot coexist with another.
+#[napi]
+pub fn register_lancedb_metrics_recorder() -> bool {
+    lancedb::metrics_otel::register_metrics_recorder()
+}
+
+/// The catalog of described LanceDB metrics. Empty until the recorder is installed.
+#[napi]
+pub fn lancedb_metrics_catalog() -> Vec<MetricDescription> {
+    lancedb::metrics_otel::metrics_catalog()
+        .into_iter()
+        .map(|desc| MetricDescription {
+            name: desc.name,
+            kind: desc.kind.as_str().to_string(),
+            unit: desc.unit,
+            description: desc.description,
+        })
+        .collect()
+}
+
+/// A point-in-time snapshot of every recorded metric. Empty until the recorder
+/// is installed.
+#[napi]
+pub fn snapshot_lancedb_metrics() -> Vec<MetricPoint> {
+    lancedb::metrics_otel::snapshot_metrics()
+        .into_iter()
+        .map(MetricPoint::from)
+        .collect()
+}