Design configuration syntax for metrics with arbitrary attributes

[jfallows]

Support arbitrary attributes in telemetric metrics, with either literal values, resolver expressions, or binding-specific expressions.

For example, something like this.

binding:
  type: http
  kind: server
  options:
    ...
  routes:
    ...
  telemetry:
    metrics:
      - http.duration
    attributes:
      method: ${http.request.method}
      path: ${http.request.path}
      status: ${http.response.status}