add support for canary deployment

[hervenicol]

Towards https://github.com/giantswarm/giantswarm/issues/28584

As upstream PR for deploying canary as a deployment (grafana/loki#13362) is not moving forward, let's implement it in our chart.

[hervenicol]

Tested on golem with these values:

lokiCanary:
  enabled: true
  extraArgs:
    - "-tenant-id=golem"

3 canary pods running, and generating metrics.

It shows 33% errors, but I can't figure out why.

[hervenicol]

Well, the dashboard looks crappy, but the canary works. 🤷

[QuentinBisson]

Did you figure out why you had 33% of errors?

[QuentinBisson]

If the dashboard is shit, we can most likely improve it upstream :)

[hervenicol]

Did you figure out why you had 33% of errors?

Well, 33% was an optimistic (and possible) value.
Currently it clearly shows nonsense 🤣

But yes, I hope to investigate and fix the dashboard later, and hopefully upstream.