[receiver/oracledb] Adding 'oracledb.logons' metric

[nenadnoveljic]

Description

Adds the oracledb.logons metric to track logon operations. Logons can be costly and, in large numbers, may lead to CPU spikes and processing delays. I wrote more about this scenario here (optional read):

[dmitryax]

If we ever possible can have oracledb.logon.<something else> metric, this should be oracledb.logon.count. If not, this name is good.

See https://github.com/open-telemetry/semantic-conventions/blob/main/docs/general/naming.md#naming-rules-for-counters-and-updowncounters for more guidelines

[nenadnoveljic]

We might add the following metrics later (based on v$statname):

• oracledb.user_logons: only user logons operations (subset of oracledb.logons which counts also the background processes)
• oracledb.current_logons: current number of logons to the DB

Both are count metrics. I can't imagine any non-count metrics in this namespaces. But let me know if you nevertheless prefer a separate namespace with count, e.g. oracledb.logon.count.

SQL> select name from v$statname where name like '%logon%';

NAME
----------------------------------------------------------------
logons cumulative
logons current
user logons cumulative

[atoulme]

Please address the conflict and mark ready for review again.