Possible prom-client version conflicts

[lbeschastny]

Hi.

Currently prom-client is added to this package dependencies, but it could lead to serious issues if it's version doesn't match the one used by the service.

Instead, it should be in the package peerDependencies section, ideally with as loose version range as possible.

Something like this should probably work:

  "peerDependencies": {
    "prom-client": "14.x || 15.x"
  }