[#6468] feature(core): add metrics for mysql backend connect pool

[TEOTEO520]

What changes were proposed in this pull request?

Support metrics for JDBC backend connect pool :

1. activeConnectionCount
2. idleConnectionCount
3. maxConnectionCount

Why are the changes needed?

Support metrics for JDBC connection pool

Fix: #6468

Does this PR introduce any user-facing change?

No

How was this patch tested?

No need to test

[TEOTEO520]

Thank you for reviewing

[FANNG1]

@TEOTEO520 Generally LGTM, could you fix the ci failures?

[tengqm]

why change underscores to dashes?

[TEOTEO520]

Other metrics in MetricNames also use dashes:

[tengqm]

where are these from?

[tengqm]

to be clear, I came from the Prometheus world. https://prometheus.io/docs/practices/naming/

[tengqm]

The community practices for database metrics: https://opentelemetry.io/docs/specs/semconv/database/database-metrics/

[FANNG1]

the metrics names will be converted to Prometheus format when exporting to prometheus, like a.b.c-d will be converted to a_b_c_d

[tengqm]

Thanks for the pointer. It is good to have such a converter, but can we simply avoid the - in the first place? It is dangerous and annoying if people want to write some expressions. - can be interpreted as minus.

[FANNG1]

It's usual to use - as the seq for names, and mostly the name doesn't contain expressions.

[FANNG1]

@tengqm is there any other comments?

[tengqm]

I am still more leaning towards removing the - from metric names. But it is not a strong opinion.

[FANNG1]

I am still more leaning towards removing the - from metric names. But it is not a strong opinion.

I don't have strong option to using - or _ as metrics name separator,as Graviitno use - not _ for configuration name seperator, and for compatibility, how about keep current style?

[jerryshao]

Since we already use - for the metrics name, also prometheus is not the only metrics sink we support, so I would like to still use - to keep the consistency.