Consider returning the metric on duplicated registration, Error::AlreadyReg

[sunng87]

This is similar request to #495

Currently, if we try to register the same metric to registry, we get an error and there is no way to fetch the metric from registry. This make it impossible to reuse the metric. Of course we can get before register but in concurrent scenario it can break.

I haven't looked very deep into the code, but the idea seems to be returning the already registerd metric as a member of Error::AlreadyReg. Also we need to update those macros to ensure that every time we can the macro with legal input, it should return a metric handle.

Context: we hit this issue with opendal apache/opendal#4854 in greptimedb GreptimeTeam/greptimedb#2861

Let me know what do you think. Thank you!

[lucab]

Thanks for the report. If I'm understanding correctly, this request is an exact duplicate of #495. Can we close this duplicate and keep the discussion there? Or is there some important detail that is different here?

[sunng87]

Yes, please. Let me add my comment to #495