Add logging relation to tensorboards-web-app

[rgildein]

Use LogForwarder from loki_push_api library to forwards all logs from workload container to Loki. This requires to use ops 2.9 or newer, so it was needed to be updated.

fixes: #111

How I tested it:

# deployed the cos following https://charmhub.io/topics/canonical-observability-stack/tutorials/install-microk8s
$ tox -e integration -- --keep-models --model kubeflow
$ juju switch test-charm-zk9n
u1-k8s:admin/test-charm-kee3 -> u1-k8s:admin/test-charm-zk9n
$ juju consume u1-k8s:cos.loki-logging 
Added u1-k8s:admin/cos.loki-logging as loki-logging
$ juju integrate grafana-agent-k8s loki-logging
$ kubectl get services -n test-charm-zk9n | grep tensorboards-web-app
tensorboards-web-app-endpoints   ClusterIP      None             <none>        <none>                                  7m23s
tensorboards-web-app             ClusterIP      10.152.183.233   <none>        5000/TCP                                7m23s
$ curl 10.152.183.233:5000
{"log":"No user detected.","status":401,"success":false,"user":null}

And after checking the Grafana, I was able to see all logs.