Prometheus endpoint cannot be validated by local json schema

[claudio-benfatto]

Describe the bug

The current v0.38.0-beta.2 release has introduced prometheus metrics and the following configuration stanza:

serve:
  prometheus:
    port: 9000
    host: localhost
    metrics_path: /metrics

however the local config.schema.json does not contain it.
This is also the schema that is loaded on startup root.go which causes the command to fail with a validation error:

INFO[0000] Config file loaded successfully.              path=/etc/auth-oathkeeper/templates/oathkeeper.yaml
FATA[0000] The configuration is invalid and could not be loaded.  [config_key=serve]="additionalProperties \"prometheus\" not allowed" config_file=/etc/auth-oathkeeper/templates/oathkeeper.yaml

Reproducing the bug

Steps to reproduce the behavior:

1. create a config.yaml file

serve:
  prometheus:
    port: 9090
    host: localhost
    metrics_path: /metrics
  proxy:
    port: 4455
  api:
    port: 4456
log:
  level: debug
access_rules:
  repositories: []
mutators:
  header:
    enabled: true
    config:
      headers:
  noop:
    enabled: true

authorizers:
  allow:
    enabled: true
  deny:
    enabled: true
  keto_engine_acp_ory:
    enabled: true
    config:
      base_url: http://auth-keto:4444/
      required_action: ""
      required_resource: ""

authenticators:
  anonymous:
    enabled: true
    config:
      subject: guest
  jwt:
    enabled: false

2. Run docker run -v ${PWD}/config.yaml:/etc/config.yaml oryd/oathkeeper:v0.38.1-alpine serve --config /etc/config.yaml

3. Process start fails with:

time="2020-05-14T09:33:05Z" level=info msg="Config file loaded successfully." path=/etc/oathkeeper.yaml
time="2020-05-14T09:33:05Z" level=fatal msg="The configuration is invalid and could not be loaded." [config_key=#]="validation failed" [config_key=mutators.header]="oneOf failed" [config_key=serve]="additionalProperties \"prometheus\" not allowed" config_file=/etc/config.yaml

Expected behavior
The prometheus server should bind to localhost:9090

INFO[0000] No tracer configured - skipping tracing setup
INFO[0000] TLS has not been configured for proxy, skipping
INFO[0000] Listening on http://:4455
INFO[0000] Listening on http://localhost:9090
INFO[0000] TLS has not been configured for api, skipping
INFO[0000] Listening on http://:4456
^CINFO[0007] HTTP server was shutdown gracefully
INFO[0007] HTTP(s) server was shutdown gracefully
INFO[0007] HTTP server was shutdown gracefully

Environment

Docker, image oryd/oathkeeper:v0.38.1-alpine

Additional context

I opened a PR with a proposed fix