Add Topic Reader buffer and commit lag gauges

[KirillKurdyukov]

Dependency

Second increment after #746, which adds double-gauge support. This PR is draft and based on double-gauge-registration; merge #746 first, then retarget this PR to master.

Summary

• Add ydb.topic.reader.local_buffer.messages, ydb.topic.reader.local_buffer.message_age.max and ydb.topic.reader.commit_offset.lag.max, matching C# names, units and descriptions.
• Observe existing partition/SyncReader buffers, including pending decompression and zero-byte messages.
• Measure oldest retained batch age from response receipt in fractional seconds.
• Track maximum accepted-request versus acknowledged commit-offset lag for single and bulk commits.
• Preserve InitResponse registration, closeAll cleanup and Reader business behavior.

Double-gauge API/backend changes are only in #746. Commit counters, Reader histograms and session errors remain separate increments.

Verification

• Changed-file Java diagnostics and Checkstyle are clean.
• The combined source passed the affected clean package: 53 tests, zero failures/errors/skips.
• Existing SDK/backend entrypoints passed directly: 23 tests.
• Independent gate review approved the source without blockers.

[codecov]

Codecov Report

❌ Patch coverage is 93.65079% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.15%. Comparing base (1237d01) to head (022f065).

Files with missing lines	Patch %	Lines
...java/tech/ydb/core/metrics/OpenTelemetryMeter.java	66.66%	1 Missing and 2 partials ⚠️
...ain/java/tech/ydb/topic/read/impl/ReadSession.java	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #745      +/-   ##
============================================
+ Coverage     75.04%   75.15%   +0.10%     
- Complexity     3683     3708      +25     
============================================
  Files           394      394              
  Lines         16623    16679      +56     
  Branches       1755     1757       +2     
============================================
+ Hits          12475    12535      +60     
+ Misses         3531     3530       -1     
+ Partials        617      614       -3     

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