feat: add metrics; fix latency check; latency time in sec

[y-eight]

Motivation

Implements #32 and fixes the latency tests.

Checks can register Prometheus collectors at sparrow lvl. Therefore a new Interface func has been added to the Checks interface.

The latency time has been changed from Microseconds to Seconds. The default & best practice for promethus is seconds. This will affect the API as well.

Changes

For additional information look at the commits.

[y-eight]

rdy for review

[y-eight]

@ALL should we separate the checks in dedicated packages e.g. checks/health, checks/latency? checks files are getting bigger, we could outsource e.g the metrics part in a separate file.

[lvlcn-t]

should we separate the checks in dedicated packages e.g. checks/health, checks/latency? checks files are getting bigger, we could outsource e.g the metrics part in a separate file.

@y-eight You're right, checks are getting heavy. Breaking it down into checks/health and checks/latency sounds like a good plan to keep things simple.

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Additionally thanks for fixing the rest of the latency times that were incorrect.

[puffitos]

I've pushed some commits regarding doc grammar and I've changed a docstring. I only have some questions and concerns regarding concurrency in some functions.

Otherwise than that, everything seems OK.