Add OpenTelemetry metrics monitor (faust[opentelemetry])

[wbarnha]

Description

Adds OpenTelemetryMonitor, a sensor that reports the same metric set as the built-in Statsd and Datadog monitors, but through the OpenTelemetry metrics API — so Faust metrics can be exported to any OpenTelemetry-compatible backend (OTLP, Prometheus, the console, ...).

This is the metrics counterpart to the OpenTelemetry tracing discussions in #688 / #689, and is intentionally standalone: it depends on neither of those PRs.

How it works

• Faust depends only on opentelemetry-api, via the new optional faust[opentelemetry] extra. Every instrument is a cheap no-op until your application configures a global MeterProvider with the exporter of its choice — the library never picks an SDK or exporter for you.
• Following OpenTelemetry conventions, each instrument is dimensioned by attributes (topic, partition, stream, table, status_code, ...) rather than baking those into the metric name the way Statsd does (read_offset.{topic}.{partition}).
• Modeled on the existing DatadogMonitor (same Monitor hooks, same metric coverage), so it stays consistent with the other backends and calls super() first to preserve the in-memory Monitor stats.

Usage:

from opentelemetry import metrics
from opentelemetry.sdk.metrics import MeterProvider
from opentelemetry.sdk.metrics.export import (
    ConsoleMetricExporter, PeriodicExportingMetricReader,
)
from faust.sensors.otel import OpenTelemetryMonitor

reader = PeriodicExportingMetricReader(ConsoleMetricExporter())
metrics.set_meter_provider(MeterProvider(metric_readers=[reader]))

app = faust.App('example', broker='kafka://localhost:9092')
app.monitor = OpenTelemetryMonitor()

Instruments

• Counters: faust.messages.received, faust.events.total, faust.messages.sent, faust.messages.send_errors, faust.table.operations, faust.assignments, faust.http.requests, faust.custom.count.
• UpDownCounters (in-flight work): faust.messages.active, faust.events.active, faust.rebalances.active, faust.rebalances.recovering.
• Histograms (unit ms): faust.events.runtime, faust.commit.latency, faust.send.latency, faust.send.error_latency, faust.assignment.latency, faust.rebalance.return_latency, faust.rebalance.end_latency, faust.http.latency.
• Gauges (last-known value per topic/partition): faust.offset.read, faust.offset.committed, faust.offset.end, faust.producer.buffer.

Changes

• faust/sensors/otel.py — OTelMetrics instrument container + OpenTelemetryMonitor.
• requirements/extras/opentelemetry.txt (opentelemetry-api/-sdk >=1.23.0, for the synchronous Gauge), the opentelemetry bundle in setup.py, and the extra added to requirements/test.txt.
• tests/unit/sensors/test_otel.py — 17 tests driving the Monitor hooks against an in-memory SDK metric reader (counter/up-down/histogram/gauge points + attributes), plus the no-OpenTelemetry ImproperlyConfigured path.
• Docs — user-guide section in docs/userguide/sensors.rst, a bundle entry in docs/includes/installation.txt, and a reference autodoc page.

Verification

• pytest tests/unit/sensors/ — 156 passed (17 new), the full sensors suite green.
• black / isort / flake8 (repo pins) clean on the new files.

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.06%. Comparing base (706b502) to head (20e563e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #746      +/-   ##
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+ Coverage   95.98%   96.06%   +0.07%     
==========================================
  Files         103      104       +1     
  Lines       11069    11249     +180     
  Branches     1191     1202      +11     
==========================================
+ Hits        10625    10806     +181     
+ Misses        350      349       -1     
  Partials       94       94              

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