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/lancedb/index.ts

@@ -20,6 +20,11 @@ import { HeaderProvider } from "./header";
 // Re-export native header provider for use with connectWithHeaderProvider
 export { JsHeaderProvider as NativeJsHeaderProvider } from "./native.js";
 
+// OpenTelemetry metrics bridge. Only the high-level entry point is public; the
+// underlying recorder/catalog/snapshot functions remain internal plumbing that
+// `otel.ts` consumes from the native module.
+export { instrumentLanceDbMetrics } from "./otel";
+
 export {
   AddColumnsSql,
   ConnectionOptions,

nodejs/lancedb/otel.ts

@@ -0,0 +1,137 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+import {
+  type Attributes,
+  type MeterProvider,
+  type ObservableResult,
+  metrics,
+} from "@opentelemetry/api";
+
+import {
+  lancedbMetricsCatalog,
+  registerLancedbMetricsRecorder,
+  snapshotLancedbMetrics,
+} from "./native";
+
+let instrumented = false;
+
+/**
+ * Register LanceDB metrics as OpenTelemetry observable instruments.
+ *
+ * Installs a process-global metrics recorder and creates one observable
+ * instrument per LanceDB metric (currently object store request counts, bytes,
+ * latency, errors, and throttles) on the given (or global) `MeterProvider`. The
+ * configured `MetricReader` then collects them on its own schedule.
+ *
+ * Counters and gauges map directly to observable counters/gauges. Because
+ * OpenTelemetry has no asynchronous histogram instrument, each histogram is
+ * exported Prometheus-style as cumulative `le` bucket counts (`<name>_bucket`,
+ * with an `le` attribute) plus `<name>_count` and `<name>_sum`.
+ *
+ * Requires `@opentelemetry/api` (a dependency) and, to actually export, an
+ * OpenTelemetry SDK such as `@opentelemetry/sdk-metrics`.
+ *
+ * @param meterProvider The provider to register instruments on. Defaults to the
+ *   global provider from `@opentelemetry/api`.
+ * @returns `true` if the recorder is installed and instruments are registered.
+ *   `false` if a different `metrics` recorder is already installed in this
+ *   process (only one global recorder is permitted), in which case a warning is
+ *   emitted and no instruments are created. Calling this more than once is safe;
+ *   instruments are created only on the first successful call.
+ */
+export function instrumentLanceDbMetrics(
+  meterProvider?: MeterProvider,
+): boolean {
+  if (!registerLancedbMetricsRecorder()) {
+    console.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.",
+    );
+    return false;
+  }
+
+  if (instrumented) {
+    return true;
+  }
+
+  const provider = meterProvider ?? metrics.getMeterProvider();
+  const meter = provider.getMeter("lancedb");
+
+  const scalarCallback = (metricName: string) => (result: ObservableResult) => {
+    for (const point of snapshotLancedbMetrics()) {
+      if (point.name === metricName && point.value != null) {
+        result.observe(point.value, point.attributes);
+      }
+    }
+  };
+
+  const bucketCallback = (metricName: string) => (result: ObservableResult) => {
+    for (const point of snapshotLancedbMetrics()) {
+      if (point.name !== metricName || point.buckets == null) {
+        continue;
+      }
+      for (const bucket of point.buckets) {
+        const attributes: Attributes = {
+          ...point.attributes,
+          le: bucket.le,
+        };
+        result.observe(bucket.cumulativeCount, attributes);
+      }
+    }
+  };
+
+  const fieldCallback =
+    (metricName: string, field: "count" | "sum") =>
+    (result: ObservableResult) => {
+      for (const point of snapshotLancedbMetrics()) {
+        if (point.name !== metricName) {
+          continue;
+        }
+        const value = point[field];
+        if (value != null) {
+          result.observe(value, point.attributes);
+        }
+      }
+    };
+
+  for (const desc of lancedbMetricsCatalog()) {
+    const unit = desc.unit ?? "";
+    if (desc.kind === "counter") {
+      const counter = meter.createObservableCounter(desc.name, {
+        unit,
+        description: desc.description,
+      });
+      counter.addCallback(scalarCallback(desc.name));
+    } else if (desc.kind === "gauge") {
+      const gauge = meter.createObservableGauge(desc.name, {
+        unit,
+        description: desc.description,
+      });
+      gauge.addCallback(scalarCallback(desc.name));
+    } else if (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.
+      const bucket = meter.createObservableCounter(`${desc.name}_bucket`, {
+        description: `${desc.description} (cumulative buckets)`,
+      });
+      bucket.addCallback(bucketCallback(desc.name));
+
+      const count = meter.createObservableCounter(`${desc.name}_count`, {
+        description: `${desc.description} (count)`,
+      });
+      count.addCallback(fieldCallback(desc.name, "count"));
+
+      const sum = meter.createObservableCounter(`${desc.name}_sum`, {
+        unit,
+        description: `${desc.description} (sum)`,
+      });
+      sum.addCallback(fieldCallback(desc.name, "sum"));
+    }
+  }
+
+  instrumented = true;
+  return true;
+}