Metrics recording: Only run request timer for valid requests

[marcesher]

Currently, we wrap every request with a with-timing call based on the request path, so that we can capture per-slice metrics. However, this has a nasty side effect of also recording metrics for any invalid requests.

We should limit the with-timing calls to only valid, non-400 requests

[marcesher]

the problem here is with the current location of the metrics call. Because the request hasn't yet completed, it doesn't have access to status information. Consequently, this will need to be decoupled from the current per-request logging, or else that information will somehow need to be made available to that function

@m3brown

[mushketyk]

I would like to work on this.
Is it still relevant?

[marcesher]

@mushketyk Thanks! Definitely still relevant. I can't promise an immediate response to pull requests, but we'll do our best.