Add Prometheus/OpenMetrics endpoint with a number of useful metrics

[LukasKalbertodt]

Closes #80

This PR adds the /~metrics endpoint (for tobira serve). This endpoint exposes several metrics in the OpenMetrics format, which are tailored to Prometheus and Grafana. Many metrics are collected on the fly when the endpoint is requested, a few others are constantly collected/counted.

To see which metrics are added, take a look at constants at the top of metrics.rs. Of course, there are many more metrics one can expose. I plan to collect ideas for those in this issue: #562

I tried to keep the commits atomic, but I would guess reviewing it all in one go is better for this PR.

Pretty screenshots of the example Dashboard:

[github-actions]

🚀 This PR was deployed at https://pr561.tobira.opencast.org. The deployment will be updated whenever someone pushes onto this PR's branch.

[LukasKalbertodt]

FWIW: I just checked why requests to /~metrics take between 20 and 50ms. In debug mode it's 9ms database operations and 32ms gathering the memory info. The latter is almost entirely in procfs as it parses the quite long /proc/self/smaps file and create a bunch of data structures. We could in theory optimize that as we don't need to parse everything and are only interested in a few lines. Buuut I rather let an external library handle the potentially undocumented format of smaps.
And well the important thing: in release mode the whole endpoint replies in 10ms, which is totally fine. If we need to, we can still optimize a bit in the future.

[JulianKniephoff]

For the record: The format of /proc/[pid]/smaps isn't undocumented. It's documented in man 5 proc for example, and besides, the files in /proc are a public kernel API AFAIK. x)

[lkiesow]

Counting requests probably do not work like this if Tobira is deployed as a cluster. Especially if you have a dynamic cluster like in a 58s environment. You only count requests per instance.

[LukasKalbertodt]

That is correct. Isn't it possible to easily add up the counters of all nodes in prometheus or grafana? Like... I don't want to have a counter in the DB for this as it would add immense overhead to every request. How do other applications do this?