Merge app_agent_receiver instance metric endpoints

[nabadger]

Currently, if we have multiple app-agent-receiver endpoints in use, there is a separate /metrics endpoint for each.

With a single endpoint the URL to scrape is:

integrations/app_agent_receiver/metrics

With multiple endpoints in use, the URL is:

integrations/app_agent_receiver/$instanceId/metric

This causes a couple of problems:

1. It becomes more tricky for the user to scrape these endpoints (since $instanceId) is fairly dynamic. I'm also not sure if this introduces issues around name conventions and url path specs (does it place restrictions on the instance-name?_

2. The resulting metrics look the same, and there's no instanceId label to differentiate between them.

I believe there is __meta__ labels provided, but I'm not too sure how I can make use of these (running the agents in k8s).

In general, is there a benefit to having separate metrics endpoints?

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I'm currently working around any issues here with some relabelling config in my ServiceMonitor to add the extra labels.