Add receiver labels

[Spaceman1701]

Proposal

It would be useful to have structured identification for receivers. Today, it's common for users to use structured names for receivers that encode something about their ownership or behavior. It'd be nice if this was more explicit and easier to query. This would be especially useful for #4962

This proposal is essentially to change receiver config so it contains a labels key (and add any useful API features to query this):

receivers:
  - name: my-useful-receiver
    labels:
      owner: my-team
      kind: heartbeat

I can think of two motivating use cases right now:

1. Conventionally defining the owner of a receiver
2. Conventionally defining the category of the receiver (suggested by @SuperQ )