Add labels functionality for service discovery

[moreinhardt]

Closes #85

This could also be used to solve #160.

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This works as intended when running the end-to-end tests and keeps backwards-compatibility.

I'd have some tests for addKnownClient() and KnownClients() that verify that

• this works with labels as well as without
• expired clients are filtered
• repeated calls to addKnownClient() with different label sets actually update the labels for that client

And some tests for the request handling and JSON parsing.

Let me know if you're interested in adding those too.

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Would be cool to see this in a next release so we don't have to rely on the custom build. 🙂

[moreinhardt]

I did not change the circleci job that only scrapes one client in the end-to-end tests. So I had to revert my change that expects two metrics.