Fix how monitoring proxy messages is handled

[kian99]

Description

Currently JIMM's dashboards shows reasonable request durations for controller level calls. Model calls however show incorrect values and are missing labels.

Digging into this, the issue is how we were monitoring proxied messages in the model proxy. When a controller response is sent back to a client, we remove the client message from a map. In this function we were monitoring the duration of the request and applying labels to include the facade and method name. The error is that we were using the response from the Juju controller to record the labels but responses from Juju don't hold this information (responses are related to requests by a message ID) so instead, use the request's facade and method name which will be populated.

The same error applied to the message duration, the response does not have its start field set, only the request does so the duration of the request was default value of time.Time (0001-01-01 00:00:00 +0000 UTC) until now.

I've modified the removeMessage function to accept a message ID rather than a whole message to make things clearer.

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

[alesstimec]

LGTM