Add Topic Reader delivered metric

[KirillKurdyukov]

Summary

• Add opt-in Topic client metrics through the existing tech.ydb.core.metrics.Meter abstraction; the default remains Meter.NOOP and metric usage is reported through Observability.
• Add the ydb.topic.reader.delivered.messages counter with unit {message} and attributes topic and consumer.
• Record delivered messages immediately before the async callback or immediately before a sync receive successfully returns a message.
• Add optional reader.name with the exact configured non-empty ReaderName. It is omitted for null/empty names; no fallback identifier is generated, so unnamed readers intentionally aggregate by the other attributes.
• Do not change Reader receive, session, or commit behavior.

Tests

• ./mvnw -pl core,topic -am clean test -DskipITs
  • Reactor build successful.
  • Core: 154 tests, 0 failures, 0 errors.
  • Topic: 99 tests, 0 failures, 0 errors, 5 integration tests skipped because no local YDB helper was available.
  • Checkstyle: 0 violations.
• ./mvnw -pl topic -am -DskipITs -Dtest=ReaderMetricsTest -Dsurefire.failIfNoSpecifiedTests=false test
  • 1 focused test verifies that one message successfully returned by the sync Reader increments the delivered counter from 0 to 1.

[codecov]

Codecov Report

❌ Patch coverage is 71.05263% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.07%. Comparing base (98aab78) to head (80fdba0).

Files with missing lines	Patch %	Lines
...n/java/tech/ydb/topic/read/impl/ReaderMetrics.java	70.58%	1 Missing and 4 partials ⚠️
...va/tech/ydb/topic/impl/TopicClientBuilderImpl.java	60.00%	1 Missing and 1 partial ⚠️
...java/tech/ydb/topic/read/impl/AsyncReaderImpl.java	50.00%	2 Missing ⚠️
...main/java/tech/ydb/topic/read/impl/ReaderImpl.java	50.00%	2 Missing ⚠️

❌ Your patch check has failed because the patch coverage (71.05%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #715      +/-   ##
============================================
- Coverage     73.11%   73.07%   -0.05%     
- Complexity     3566     3572       +6     
============================================
  Files           391      392       +1     
  Lines         16497    16531      +34     
  Branches       1730     1733       +3     
============================================
+ Hits          12062    12080      +18     
- Misses         3817     3825       +8     
- Partials        618      626       +8     

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