CNDB-12716: Metric for writes blocked on memory allocation

[MSpryszynski]

What is the issue

We should know whenever writes get blocked due to memory shortage in the memtable.
CC already has a Timer metric, MemtablePool::blockedOnAllocating. However, the problem with a Timer metric is that it is only updated once the timed event ends. I.e. when the writes are unblocked.

What does this PR fix and why was it fixed

Added counter metric for writes blocked on allocation to be able to see spikes of blocked allocations immediately

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits
• All new files should contain the DataStax copyright header instead of the Apache License one

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-2279 rejected by Butler

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5 regressions found
See build details here

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Found 5 new test failures

Test	Explanation	Runs	Upstream
o.a.c.distributed.test.AbortedQueryLoggerTest.testLogsReadMetrics	REGRESSION	🔴	0 / 30
o.a.c.index.sai.cql.VectorCompaction100dTest.testZeroOrOneToManyCompaction[version=ec enableNVQ=false]	REGRESSION	🔴	0 / 30
o.a.c.index.sai.cql.VectorKeyRestrictedOnPartitionTest.partitionRestrictedTest[version=ec enableNVQ=false enableFused=false] (compression)	REGRESSION	🔴	0 / 30
o.a.c.index.sai.cql.VectorSiftSmallTest.testCompaction[version=fa enableNVQ=false enableFused=false]	REGRESSION	🔴	0 / 30
o.a.c.index.sai.cql.VectorTypeTest.tracingTest[version=ca enableNVQ=false enableFused=false]	REGRESSION	🔴	0 / 30

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Found 4 known test failures