Mixin: Add Query Frontend grafana dashboard

[jessicalins]

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Hey! 👋
I've added a grafana dashboard for the Query Frontend component. I used the Query/Store dashboards as base.
Here is a screenshot of how it currently looks like:

I'm not sure if all metrics that are interesting are represented in the dashboard, but I've tried to add some basic ones (RED metrics/caching/resources)

Verification

I've ran make examples to generate the query_frontend.json file and I've imported it to Grafana to get an idea how it looks like.

[jessicalins]

I saw that we would like to fix the naming convention to e.g. queryFrontend (this TODO: https://github.com/thanos-io/thanos/blob/main/mixin/config.libsonnet#L52)
I've tried at first to use it like that, but when running make examples and when it tries to generate the json file, I get:

/home/jessica/go/bin/jsonnet-v0.17.0 -J mixin/vendor -m examples/dashboards mixin/dashboards.jsonnet
RUNTIME ERROR: Field does not exist: query_frontend

Does someone have an idea of what it could be?

[saswatamcode]

I was able to replicate this error by changing everything to queryFrontend except the query_frontend.json in this line. Once changed to queryFrontend.json, make examples runs successfully!

I think this is due to how examples are generated by dashboards.jsonnet.

[jessicalins]

thanks, that solved the problem!
since I'm already touching those files, should I also update https://github.com/thanos-io/thanos/blob/main/mixin/config.libsonnet#L52 to this convention in this PR? or better in another one?

[saswatamcode]

Yes, it would be better to do the update in a separate PR! 🙂

[jessicalins]

thanks, added here: #4859

[bwplotka]

Amazing work!

Let's merge and iterate, looks good! Thanks 💪🏽