Enable service graph processor

[PietroPasotti]

it works

but the loki/prom datasource UIDs are hardcoded in the coordinator charm.

Reason for that is ATM we have no way to determine the datasource UIDs for 'other' datsources than ourselves.
We likely need a new relation interface or a clever solution.

[PietroPasotti]

TODO:

• add a 'datasource_uid' field for grafana to tell the datasource provider what its DS UID is
• add the "datasource_uid" field to the remote_write interface so that prometheus/mimir can forward to Tempo "here's my DS UID" - add the "datasource_uid" field to the loki_push_api interface so that loki can forward to Tempo "here's my DS UID"
• add the "datasource_uid" field to the tracing interface so that tempo can forward to whoever needs it in the future "here's my DS UID"

[PietroPasotti]

update: we're propagating the datasource UIDs to the sources themselves as planned, but instead of sending the uids through remote_write and loki_push_api, we're adding a new grafana_source_correlate interface that we can use to relate loki and prom to tempo (and in the future if needed, loki and prom to one another.

[michaeldmitry]

Testing Instructions

1. deploy Tempo HA
2. deploy grafana-k8s
3. deploy prometheus-k8s
4. juju integrate tempo grafana:grafana-source
5. juju integrate prometheus grafana:grafana-source
6. juju integrate tempo prometheus:receive-remote-write
7. juju integrate tempo prometheus:send-datasource
8. login to grafana UI
9. go to sources -> Tempo -> service graphs, you should see the service graph enabled and at least 2 nodes shown in the graph

TODO

• rebase to main after Datasource exchange #86 is merged
• merge Implement grafana_datasource_exchange prometheus-k8s-operator#654

[PietroPasotti]

it's looking good, a little bit of polishing here and there and then it's good to go

[PietroPasotti]

@michaeldmitry FYI I can't approve it because I'm the owner of the PR technically