fix(metrics): support new micrometer prometheus package and make pushgateway optional

[utafrali]

Fixes #16109

Ran into this while setting up Dubbo 3.3.6 with Spring Boot 3.5 and dubbo-observability-spring-boot-starter. Prometheus metrics were silently not initializing because MetricsSupportUtil.isSupportPrometheus() was checking for class names that no longer exist in Micrometer 1.13+ (io.micrometer.prometheus.PrometheusConfig) and requiring PushGateway exporter classes that aren't always on the classpath.

The detection now accepts both the old io.micrometer.prometheus and the new io.micrometer.prometheusmetrics package. PushGateway classes are no longer required for the check since most setups just use the scrape endpoint.

Also wrapped PushGateway initialization in PrometheusMetricsReporter with a NoClassDefFoundError catch so the reporter still works for scrape mode when pushgateway deps are missing. Broadened the error handling in PrometheusMetricsReporterFactory.dependenciesNotFound() to properly catch any micrometer or prometheus class loading failures.

Tests:

mvn -pl dubbo-metrics/dubbo-metrics-api -Dtest=MetricsSupportUtilTest test
mvn -pl dubbo-metrics/dubbo-metrics-prometheus -Dtest=PrometheusMetricsReporterTest,PrometheusMetricsReporterFactoryTest test
mvn -pl dubbo-config/dubbo-config-api -am -Dtest=DefaultApplicationDeployerTest test

All passing locally on JDK 21.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.63636% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.79%. Comparing base (bd2ed2a) to head (96a0722).
⚠️ Report is 16 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
...s/prometheus/PrometheusMetricsReporterFactory.java	0.00%	4 Missing ⚠️
.../metrics/prometheus/PrometheusMetricsReporter.java	80.00%	3 Missing ⚠️
...apache/dubbo/metrics/utils/MetricsSupportUtil.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #16128      +/-   ##
============================================
- Coverage     60.79%   60.79%   -0.01%     
+ Complexity    11754    11752       -2     
============================================
  Files          1953     1953              
  Lines         89118    89122       +4     
  Branches      13444    13443       -1     
============================================
- Hits          54182    54178       -4     
- Misses        29368    29375       +7     
- Partials       5568     5569       +1     

Flag	Coverage Δ
integration-tests-java21	32.12% <9.09%> (-0.05%)	⬇️
integration-tests-java8	32.24% <9.09%> (+0.02%)	⬆️
samples-tests-java21	32.13% <9.09%> (-0.02%)	⬇️
samples-tests-java8	29.71% <9.09%> (-0.04%)	⬇️
unit-tests-java11	59.03% <63.63%> (-0.03%)	⬇️
unit-tests-java17	58.52% <63.63%> (-0.03%)	⬇️
unit-tests-java21	58.55% <63.63%> (-0.03%)	⬇️
unit-tests-java25	58.51% <63.63%> (+<0.01%)	⬆️
unit-tests-java8	59.04% <63.63%> (-0.01%)	⬇️

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[SavitarC]

Thanks for the contribution. I’m not sure whether dropping support for the new PushGateway version directly would be the most appropriate approach.
@zrlw plz check my PR and see if it might be a suitable way forward? #16132

[utafrali]

Thanks for the pointer to #16132 — the adapter approach there is more thorough than what I have here.

Just to clarify the scope of this PR in case it's useful: it doesn't drop PushGateway support. It only stops requiring the PushGateway classes inside MetricsSupportUtil.isSupportPrometheus() and wraps the initializer in a NoClassDefFoundError catch, so scrape-only setups stop silently failing when PushGateway isn't on the classpath. When the PushGateway deps are present it still initializes the exporter normally.

That said, #16132 covers proper dual-stack detection and both legacy/new PushGateway paths, which supersedes this. If that one lands I'm happy to close this out; leaving it open for now only in case a smaller interim patch is useful.