Play Framework

feat: Micrometer metrics with Prometheus exposition at /@metrics (PF-13)

Adds Micrometer-based metrics with a Prometheus scrape endpoint:

- play.libs.Metrics — static facade for application code (counter, timer,
  gauge). Default registry is a SimpleMeterRegistry so call sites work even
  when MetricsPlugin is disabled; the plugin swaps in a
  PrometheusMeterRegistry on onApplicationStart and restores SimpleMeterRegistry
  on onApplicationStop. install() is public so plugins outside play.libs can
  swap the registry.

- play.plugins.MetricsPlugin (slot 30 in play.plugins) — reads
  metrics.enabled (default true) and metrics.basePath (default /@metrics).
  Auto-registers Micrometer's bundled JVM binders (memory, GC, threads,
  classloader). rawInvocation short-circuits the configured base path with
  Content-Type: text/plain; version=0.0.4; charset=utf-8 (Prometheus text
  format) and registry.scrape() body.

- HTTP request metrics: ActionInvoker.invoke stamps a start nanotime and,
  in the existing finally block adjacent to the PF-9 MDC clear, records
  duration into the Timer http_server_requests tagged with method, status,
  route. Tagging falls back to request.path when request.action is unset
  (unrouted requests). Metric recording is wrapped in a try/throwable so
  a misbehaving registry can never break request handling.

- Dependencies: micrometer-core, micrometer-commons, micrometer-observation,
  micrometer-registry-prometheus 1.13.6 plus the io.prometheus
  prometheus-metrics-* family at 1.2.1 (the new client_java v1, NOT the
  legacy simpleclient line — Micrometer 1.13.x switched). HdrHistogram and
  LatencyUtils are micrometer-core runtime transitives. The fork's resolver
  pre-pins declared modules so jars were pre-seeded into framework/lib/
  before ant resolve.

- application-skel/conf/application.conf documents both keys (commented out
  so defaults apply).

OTLP push exporter is intentionally deferred — Prometheus scrape is the
baseline; OTLP can be layered as a follow-up ticket without affecting the
Metrics facade or the plugin lifecycle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>