Allow for scoping of auto marking

[domtronn]

The automark functionality should be allowed to take a scope to be used in different areas of the application, rather than just once on access.

e.g.

[
  {
    "name": "total_upstream_requests",
    "help": "A count of the upstream calls with cache hits",
    "type": "Gauge",
    "mark": {
      "id":"cache",
      "method":"set",
      "path":["cache", "size"]
     }
  },
  {
    "name": "http_request_duration_hist_seconds",
    "help": "The response time of requests",
    "type": "Histogram",
    "labelNames": [
      { "key": "method", "path": [ "req", "method" ] },
      { "key": "code", "path": [ "res", "status" ] },
      { "key": "path", "path": [ "_matchedRoute" ] }
    ],
    "mark": {
      "id": "access",
      "method": "observe",
      "path": [ "res", "responseTime" ]
    },
    "buckets": {
      "type": "linear",
      "start": 40,
      "buckets": 15,
      "factor": 40
    }
  }
]

This declare to metrics that will be marked given different contexts, one on access and one when interacting with the cache, you could call them with

meters.automark('cache', cache)
meters.automark('access', ctx)

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One of the main concerns would be how to make this backwards compatible for when no id / namespace is defined.