apiserver/observer: Logging of Ping requests & replies

[mjs]

Description of change

To reduce log noise, ping requests and replies weren't being logged but
this makes it more difficult to debug ping related issues. Pings are
now logged to a separate logger (juju.apiserver.ping) and are logged
at TRACE. This means they aren't included by default but can be
observed when required.

QA steps

Bootstrap a controller and deploy some units. Wait a while and observe that no ping logs are seen and that other request/reply logging works as before.

Increase the log level on juju.apiserver.ping:

$ juju model-config -m controller logging-config='juju.apiserver.ping=TRACE;<root>=INFO;juju=DEBUG;unit=DEBUG'

Observe that juju.apiserver.ping logs are seen:

$ juju debug-log -m controller --replay | grep -i ping
...
machine-0: 23:29:03 TRACE juju.apiserver.ping <- [13] unit-ubuntu-0 {"request-id":161,"type":"Pinger","version":1,"request":"Ping","params":{}}
machine-0: 23:29:03 TRACE juju.apiserver.ping -> [13] unit-ubuntu-0 {"request-id":161,"response":{}}
...

Documentation changes

N.A.

Bug reference

N.A.

[wallyworld]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju