Add metric for currently-in-flight HTTP requests

[evankanderson]

Changes

Adds an http.server.requests metric to count in-flight HTTP requests on the server.

I was starting to implement some monitoring using the HTTP semantic conventions, and realized that it was also useful to have a count of the current in-flight HTTP requests (those which had started but not yet completed). This allows monitoring both the concurrency of the server and detecting unfortunate queueing or backlog events before the requests have completed (for example, if the request is blocked indefinitely).

(Will update CHANGELOG.md once I have a PR #)

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[evankanderson]

(Merged CHANGELOG.md -- let me know if I should squash)

[evankanderson]

Is this a reasonable addition? Do I need to bring this up at a contributor meeting?

[anuraaga]

Thanks!

[reyang]

Please see my comment here.

[jmacd]

@reyang In other cases, we've discussed how it may be acceptable to specify semantic conventions without saying how they should be generated. For the case of "in flight HTTP requests" we could either materialize a dedicated metric or we could calculate the number in-flight by subtracting one metric from another. It's clearly much harder to do the second, so most users will prefer the first. But if you have a sophisticated metric system, it's redundant to record an additional metric and you shouldn't have to.

The example we discussed was metrics with .utilization, .usage, and .limit suffixes. With any two of these you can calculate the third, so which should we record? All three are useful to monitor, so it's hard to argue that in some cases you shouldn't just record all three.

I agree with what @jmacd explained here #1378 (comment)

[evankanderson]

Sorry for the delay, the extra comments caught me in the middle of vacation/school break, and it took some time to dig out.

[reyang]

LGTM.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[evankanderson]

I'm not authorized to merge this PR, but it looks like it does have the requisite reviews. What's the next step?

[evankanderson]

@jmacd -- I think this is ready for merge?

[jmacd]

@evankanderson sorry for the long delay. Thank you for this contribution!