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 57.14286% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.44%. Comparing base (181e39c) to head (a37ba16).
⚠️ Report is 4 commits behind head on topic-session-errors.

Files with missing lines	Patch %	Lines
...n/java/tech/ydb/topic/read/impl/ReaderMetrics.java	67.85%	8 Missing and 1 partial ⚠️
...ch/ydb/topic/read/impl/ReadPartitionCommitter.java	62.50%	5 Missing and 1 partial ⚠️
...ain/java/tech/ydb/topic/read/impl/ReadSession.java	40.00%	6 Missing ⚠️
...a/tech/ydb/topic/read/impl/ReadPartitionQueue.java	20.00%	4 Missing ⚠️
.../java/tech/ydb/topic/read/impl/SyncReaderImpl.java	20.00%	4 Missing ⚠️
...n/java/tech/ydb/topic/read/impl/ReadPartition.java	25.00%	3 Missing ⚠️
.../main/java/tech/ydb/topic/read/impl/BatchMeta.java	71.42%	2 Missing ⚠️
...ain/java/tech/ydb/topic/read/impl/MessageImpl.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##             topic-session-errors     #775      +/-   ##
==========================================================
- Coverage                   75.48%   75.44%   -0.05%     
- Complexity                   3764     3776      +12     
==========================================================
  Files                         396      396              
  Lines                       16869    16943      +74     
  Branches                     1787     1792       +5     
==========================================================
+ Hits                        12734    12782      +48     
- Misses                       3515     3545      +30     
+ Partials                      620      616       -4     

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