Add Topic Reader metrics

[KirillKurdyukov]

Add commit counters and lag, buffered-message count and oldest-message age.

Depends on #747.

[codecov]

Codecov Report

❌ Patch coverage is 60.27397% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.42%. Comparing base (181e39c) to head (7dfb60e).
⚠️ Report is 4 commits behind head on topic-session-errors.

Files with missing lines	Patch %	Lines
...ain/java/tech/ydb/topic/read/impl/ReadSession.java	42.10%	11 Missing ⚠️
...n/java/tech/ydb/topic/read/impl/ReaderMetrics.java	78.26%	5 Missing ⚠️
...n/java/tech/ydb/topic/read/impl/ReadPartition.java	57.14%	3 Missing ⚠️
...a/tech/ydb/topic/read/impl/ReadPartitionQueue.java	0.00%	3 Missing ⚠️
...main/java/tech/ydb/topic/read/impl/ReadConfig.java	71.42%	2 Missing ⚠️
.../java/tech/ydb/topic/read/impl/SyncReaderImpl.java	33.33%	2 Missing ⚠️
.../main/java/tech/ydb/topic/read/impl/BatchMeta.java	50.00%	1 Missing ⚠️
...ain/java/tech/ydb/topic/read/impl/MessageImpl.java	0.00%	1 Missing ⚠️
...ch/ydb/topic/read/impl/ReadPartitionCommitter.java	87.50%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (60.27%) 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                     @@
##             topic-session-errors     #775      +/-   ##
==========================================================
- Coverage                   75.48%   75.42%   -0.07%     
- Complexity                   3764     3769       +5     
==========================================================
  Files                         396      396              
  Lines                       16869    16932      +63     
  Branches                     1787     1793       +6     
==========================================================
+ Hits                        12734    12771      +37     
- Misses                       3515     3543      +28     
+ Partials                      620      618       -2     

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