365 akka http connection metrics

[lgajowy]

Closes #365

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Summary:

• adds http connections metric with it's config
• adds grafana dashboard for it

[lgajowy]

@worekleszczy Some unit/integration testing is still needed - i need to figure out how to do it best

[lgajowy]

@worekleszczy Regarding adding testing to this pr - it seems to be a bigger research than I initially anticipated:

• maybe it's possible to use the context similar to what you did in pr 366 path attribute updated #402. if we can use otel context to assert the fact that some connection-related attributes appeared, that could lead us somewhere. Sounds a little bit hacky in this particular case, though.

• it would be a lot better to be have some test MetricsSDK used by the AgentForTesting. The instrument instances (eg. connections counter) would assert if the value was passed to them in the test. No idea if this is doable.

• I noticed that there are methods like below in the AgentTestRunner:

testRunner.runWithClientSpan()
testRunner.runWithServerSpan()

this looks like something useful to assert if the span was created correctly. We do not use spans however but maybe there's something similar for metrics alone?

[worekleszczy]

LGTM