Send writes without consistency due to missing coordinator write metr…

[serban21]

…ics at table level before Cassandra 4. Also add support for C*4 CoordinatorWriteLatency

Closes #63

[zegelin]

@serban21 Thanks for the PR.

I'll have to give some more thought to this before I merge it. I made some changes late last year to explicitly exclude read and write metrics, since they are aggregates of the per-consistency-level metrics.

[serban21]

@zegelin yes, I saw that. I wrote on the PR why I think it's needed. Since then I looked a bit more into this, and I found out that ClientRequest metrics are separated by consistency level only since Cassandra 3.10 (see https://issues.apache.org/jira/browse/CASSANDRA-7384 and apache/cassandra@221f27a).

So on any version before 3.10 there is only the total write, which will be excluded due to the consistency == null test. And per-table writes are available only from Cassandra 4.

A better solution might be to check if the version is less than 3.10, and emit the write metrics only in this case. The version is available in StorageService getReleaseVersion, and it can be compared by using the CassandraVersion class. How does this sound?

[zegelin]

Sounds reasonable.

Some thought will need to be given to how multiple C* versions are supported going forward. I'd like to avoid peppering the code with version tests, where possible.