Blocked by #2331

Summary

• bound the collection spin wait and observation buffer
• coordinate collection through explicit buffer generations and phase transitions
• replay buffered observations before surfacing a collection timeout
• skip timed-out native-histogram maintenance rather than throwing from observe()
• document the generation handoff and striped-count protocol

This is stacked on the focused #2282 fix so the two Buffer concerns can be reviewed separately. It replaces the #2287 portion of #2297.

Fixes #2287

Ongoing discussion

• Should the five-second collection deadline remain an internal default, or become publicly configurable? See the earlier discussion.
• The bounded buffer currently applies backpressure rather than dropping observations. The earlier review suggested drop-on-overflow; this version avoids silent data loss but can block an observer while the generation remains active. See the earlier discussion.
• The generation/phase handoff is a substantial concurrency change and should be reviewed independently of the simple stripe-index fix.

The earlier concerns about an unbounded second wait and exceptions escaping from native-histogram maintenance are addressed by this implementation.

Validation

• mise run lint:fix
• mise run build
• ./mvnw test -pl prometheus-metrics-core -Dcoverage.skip=true -Dcheckstyle.skip=true