[platinumhamburg]

Purpose

Closes #4844.

requestProcessTimeMs includes time spent waiting for an asynchronous response future, including delayed writes. It does not show how long an RPC occupies a request worker. This change measures that synchronous interval separately.

Brief change log

• Record requestSyncProcessTimeUs from request dequeue through synchronous processing and request-buffer release, including failed requests.
• Record requestSyncProcessTimeUsSum for interval worker-time accounting by the existing request type groups.
• Reuse the same request classification for synchronous and completion metrics without adding labels or changing existing latency semantics.

Tests

• ./mvnw -pl fluss-rpc -am -Dtest=RequestProcessorMetricsTest,RequestsMetricsTest,NettyServerHandlerTest -Dsurefire.failIfNoSpecifiedTests=false test -q passed.
• Re-ran RequestProcessorMetricsTest after its final test adjustment; passed.
• The affected Maven build passed Spotless, Checkstyle, and RAT; git diff --check passed.

API and Format

Adds two server metrics. No RPC API or storage format changes.

Documentation

Updated the request metric reference with the synchronous timing boundary and its distinction from asynchronous request completion.