Introduce grafana callback plugin. (#34246)

* Introduce grafana callback plugin.

The grafana plugin plublishes annotations to the HTTP API available in
Grafana 4.6+

The callback publishes:
* An annotation on playbook start, tagged with "ansible",
 "ansible_event_start" and playbook name, example:

```
{
    "text": "Started playbook test.yml\n\nFrom 'pc45.home'\nBy user 'remirey'\n",
    "tags": ["ansible", "ansible_event_start", "test.yml"],
    "time": 1514291163000
}
```

* An annotation on error containing the host and task who failed and
  tagged with "ansible", "ansible_event_failure" and playbook name, example:

```
{
    "text": "Playbook test.yml Failure !\n\nFrom 'pc45.home'\nBy user 'remirey'\n\n'TASK: simulate failure' failed on localhost\n\ndebug: {\"changed\": false, \"msg\": \"Some random failure\"}\n",
    "tags": ["ansible", "ansible_event_failure", "test.yml"],
    "time": 1514291165000
}
```

* A region annotation emitted on playbook stats, tagged with "ansible",
  "ansible_report" and playbook name, example:

```
{
    "text": "Playbook test.yml\nDuration: 1.641703\nStatus: FAILED\n\nFrom 'pc45.home'\nBy user 'remirey'\n\nResult:\n{\"localhost\": {\"unreachable\": 0, \"skipped\": 0, \"ok\": 2, \"changed\": 1, \"failures\": 1}}\n",
    "tags": ["ansible", "ansible_report", "test.yml"],
    "isRegion": true,
    "timeEnd": 1514291165000,
    "time": 1514291163000
}
```

Fixes #34225

.github/BOTMETA.yml

@@ -935,6 +935,9 @@ files:
   lib/ansible/plugins/callback/unixy.py:
     support: community
     maintainers: akatch
+  lib/ansible/plugins/callback/grafana_annotations.py:
+    support: community
+    maintainers: rrey
   lib/ansible/plugins/cliconf/:
     maintainers: $team_networking
     labels: networking