Add logging relation to tensorboards-controller

[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: #110

How I tested it:

# deployed the cos following https://charmhub.io/topics/canonical-observability-stack/tutorials/install-microk8s
$ tox -e integration -- --keep-models
$ juju switch test-charm-98bl
$ charmcraft pack  # since there is test removing charm
Created 'tensorboard-controller_ubuntu-20.04-amd64.charm'.

Charms packed:

    tensorboard-controller_ubuntu-20.04-amd64.charm 
$ juju deploy ./tensorboard-controller_ubuntu-20.04-amd64.charm --resource tensorboard-controller-image=kubeflownotebookswg/tensorboard-controller:v1.9.0-rc.0
Located local charm "tensorboard-controller", revision 0
Deploying "tensorboard-controller" from local charm "tensorboard-controller", revision 0 on [email protected]/stable
$ juju consume u1-k8s:cos.loki-logging 
Added u1-k8s:admin/cos.loki-logging as loki-logging
$ juju integrate grafana-agent-k8s loki-logging
$ juju integrate grafana-agent-k8s tensorboard-controller:logging
$ juju integrate tensorboard-controller:gateway-info istio-pilot

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