prometheus: fix scrape duration growing unbounded (#13667)

[PrashantBhanage]

Description

This PR fixes unbounded growth of Prometheus /metrics scrape duration
against the CloudStack management server (root cause tracked in #13586,
improvements requested in #13667).

Before: the exporter's HttpServer used the JDK default single-threaded
executor, so a slow scrape serialized/queued every other scrape behind it.
Additionally, updateMetrics() had no guard against concurrent or rapid
recomputation, so scrape frequency could multiply backend load
indefinitely. Only a full management server restart reset the growth.

After: three targeted, contained changes, all inside
plugins/integrations/prometheus/:

1. Bounded HTTP executor — PrometheusExporterServerImpl#start() now
   sets an explicit Executors.newFixedThreadPool(2) as the HttpServer's
   executor instead of relying on the JDK default, and shuts it down
   cleanly in stop().
2. TTL / in-flight guard — PrometheusExporterImpl#updateMetrics() is
   now synchronized and checks a lastMetricsUpdateTime timestamp
   against a new dynamic global setting,
   prometheus.exporter.metrics.min.refresh.interval (default 5 seconds).
   Scrapes arriving within that window reuse the previously computed
   metrics instead of triggering a fresh, expensive recomputation.
3. Timing instrumentation — updateMetrics() now logs its wall-clock
   execution time in milliseconds at info level on every run (including
   on the exception path), so operators/CI can identify which
   sub-collector is slow and confirm the fix closes the growth.

This PR deliberately does not touch AlertManagerImpl or the
capacity-calculation executor in server/ — that code already creates a
fresh thread pool per invocation and reads capacity.calculate.workers
dynamically on main, and rewriting it was flagged in the issue comments
as unnecessary scope creep that introduced its own bugs (stale config,
swallowed shutdown exceptions, thread-pool leaks).

Fixes: #13667
Ref: #13586

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Feature/Enhancement Scale or Bug Severity

Bug Severity

• Major

Screenshots (if appropriate):

N/A — backend-only change, no UI impact.

How Has This Been Tested?

• Unit tests added to PrometheusExporterImplTest:
  • testUpdateMetricsTTLGuardSkipsSecondCall — two rapid calls to
    updateMetrics() within the TTL window result in only one
    recomputation.
  • testUpdateMetricsTTLGuardAllowsAfterInterval — after the configured
    interval elapses, a subsequent call triggers a fresh recomputation.
• mvn -pl plugins/integrations/prometheus test → 6/6 tests pass (4
  existing + 2 new).
• mvn -pl plugins/integrations/prometheus -am install → build success.
• Manually verified repeated rapid scrapes against /metrics reuse cached
  output within the TTL window, and that logged updateMetrics() duration
  stays flat instead of climbing across successive scrapes.

How did you try to break this feature and the system with this change?

• Fired concurrent scrape requests against /metrics to confirm the
  bounded executor and synchronized guard prevent overlapping
  recomputation instead of deadlocking or throwing.
• Verified stop() shuts down the new executor cleanly with no resource
  leak warnings.
• Confirmed the new prometheus.exporter.metrics.min.refresh.interval
  setting is dynamically adjustable at runtime without a restart, and
  that a value of 0 effectively disables the guard (falls back to
  original per-scrape recomputation behavior) for anyone who wants that.
• Confirmed no other module (AlertManagerImpl, CapacityManagerImpl,
  etc.) was touched, keeping this change fully isolated to the exporter
  plugin.