Ensure That Metrics Only Decrements Active Requests Once

[isomarcte]

Prior to this commit there was an edge case where the ops.decreaseActiveRequests would get run more than once if an error or cancellation occurred after the onFinalize method was run in onResponse.

This change adds a Ref which is initialized at the start of request handling and set to true once the ops.decreaseActiveRequests is run. All attempts to set ops.decreaseActiveRequests are guarded by this boolean value.

[isomarcte]

This may not be a bug. I'm actually not 100% sure anymore. Leaving open until I can think on it some more.

[isomarcte]

After some additional testing, I do still believe there is an edge case here and I do still think the changes I've proposed will fix it.

Here is what I know,

• I currently have a http4s 0.20.11 server running with prometheus based metrics
• Currently, the org_http4s_server_active_request value is at -1.0.

Here is what I suspect.
If cancellation or error occurs in the onEmpty handler, it is possible that the code to decrease the active request count will run twice, once in the onEmpty handler and once in either of the non-success case branches of the bracketCase release handler. This is because there is no synchronization on decrementing that value.

So, my brain is exhausted. I'll update the commit/PR notes tomorrow with some better, and hopefully actually correct, examples if not tests cases.

[ChristopherDavenport]

Bit of a hammer, does add a little weight for each request, but I think its worth it to outlaw this condition.

[rossabaker]

Oops. This is a bugfix. I should have backported this one.