Add native Micrometer metrics backend (Prometheus + optional JMX legs)

[gortiz]

Summary

Adds a new pinot-micrometer metrics plugin (pinot-plugins/pinot-metrics/pinot-micrometer) implementing the existing metrics SPI as a genuinely native Micrometer backend — a step toward adopting the de-facto JVM metrics standard.

A single Micrometer CompositeMeterRegistry natively fans each meter to its enabled, config-toggled export legs (no Dropwizard store, no mirroring):

• Prometheus leg (opt-in): a Micrometer PrometheusMeterRegistry exporting directly in Prometheus exposition format (no JMX hop), optionally over an HTTP scrape endpoint.
• JMX leg (opt-in): a Micrometer JmxMeterRegistry exposing meters as MBeans under Micrometer-native names.

The standard Micrometer JVM/system binders (memory, GC, threads, class loader, CPU, uptime, file descriptors) are bound to the composite, so basic JVM observability is exported whenever a leg is enabled.

⚠️ Metric names/resolutions intentionally differ from the legacy backends

Unlike pinot-yammer/pinot-dropwizard, this backend does not reproduce the legacy JMX-derived Prometheus names. It uses native Micrometer conventions, which are a different shape:

Metric	Legacy (Yammer/Dropwizard)	Native Micrometer
Meter	_Count, _MeanRate, _OneMinuteRate, _FiveMinuteRate, _FifteenMinuteRate (5 series, server-side EWMA)	single cumulative _total; no EWMA rates (rate computed downstream via Prometheus rate())
Timer	the meter rate set + _Min/_Max/_Mean/_StdDev + _50th…_999thPercentile	_count / _sum / _max (+ optional client-side percentiles); no EWMA rates
Gauge	_Value	a single bare series

This divergence is deliberate. Callers that need the exact legacy names will compose Yammer (→ JMX) + Micrometer (→ Prometheus/everything else) via the compound registry (a separate follow-up). The default metrics factory (Yammer) is unchanged; this backend is strictly opt-in via the factory class name.

Configuration

All keys under pinot.metrics.micrometer.:

Key	Default	Meaning
prometheus.enabled	false	enable the direct-Prometheus leg
prometheus.port	0	scrape-endpoint HTTP port (0 = no server)
prometheus.path	/metrics	scrape-endpoint path
jmx.enabled	false	enable the (Micrometer-native) JMX leg

If no leg is enabled the registry logs a WARN (meters are accepted but nothing is exported).

Implementation notes

• Gauges are registered with a strong reference to their value holder. Micrometer holds gauge state via a WeakReference by default and the SPI caller (AbstractMetrics) discards the returned wrapper, so without this gauges would silently decay to NaN after GC. Covered by a dedicated GC regression test.
• PinotMetered rate methods return NaN (no EWMA equivalent in Micrometer).
• makePinotJmxReporter is a no-op (the JmxMeterRegistry exports continuously once in the composite); addListener is a documented no-op (no native equivalent).

Testing

Native-behavior tests assert the per-type Prometheus series + values, JVM metrics presence, the HTTP scrape endpoint, NaN-safety, gauge-survives-GC, and the JMX leg's MBeans. MicrometerVsLegacyDifferenceTest instantiates both a Yammer and a Micrometer registry and characterizes the divergence (legacy meanRate() finite vs native NaN; native scrape carries the native suffixes and none of the legacy CamelCase ones). All green.

Release Notes

Adds an opt-in, native Micrometer metrics backend that exports directly to Prometheus (and/or Micrometer-native JMX) with standard JVM/system metrics. Its exported metric names/resolutions follow Micrometer conventions and differ from the legacy Yammer/Dropwizard names — it is opt-in and does not affect the default backend. Operators needing the exact legacy names can combine backends via the compound registry.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.78%. Comparing base (b27a3ad) to head (1ba1255).
⚠️ Report is 16 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #18786   +/-   ##
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  Coverage     64.78%   64.78%           
  Complexity     1309     1309           
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  Files          3380     3380           
  Lines        209544   209630   +86     
  Branches      32797    32822   +25     
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+ Hits         135746   135809   +63     
- Misses        62870    62888   +18     
- Partials      10928    10933    +5     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-21	64.78% <ø> (+<0.01%)	⬆️
temurin	64.78% <ø> (+<0.01%)	⬆️
unittests	64.78% <ø> (+<0.01%)	⬆️
unittests1	56.97% <ø> (+0.01%)	⬆️
unittests2	37.26% <ø> (-0.02%)	⬇️

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