Prometheus support

Prometheus can use Gnocchi to store its data through Remote Write Adapter. Gnocchi needs to be installed with the prometheus flavor.

Example of Prometheus configuration:

remote_write:
- url: "http://localhost:8041/v1/prometheus/write"
  basic_auth:
    username: "admin"
    password: "whatever"

The /v1/prometheus/write endpoint handles the WriteRequest protobuf message.

Gnocchi maps Prometheus metrics to its data model.

For each metric sent by Prometheus, Gnocchi maintains a corresponding resource based on each job and instance pair. This resource is created with the prometheus resource type and contains two attributes, job and instance. The metrics sent by Prometheus with this pair are attached to that resource and filled with the provided measures.