Add metrics to PromQLShardingMiddleware

[yeya24]

Is your proposal related to a problem?

Now we don't have any instrumentation to the PromQLShardingMiddleware so we might miss some very important stats.
I'd like to see metrics like Number of shardable queries and non-shardable queries.

Describe the solution you'd like

The implementation should be straightforward to add two counters to the middleware

[idoqo]

Hi @yeya24, Trying to work on this here, but not sure how to try it out Can you help provide some sample non-shardable queries?

[yeya24]

here

Hi @idoqo, I checked that branch and the code looks good! For not shardable query you could try up and shardable query you could try sum(up) by (job). Let me know if this works for you or not.