metrics: make it easier to update / discover Grafana dashboard

[marten-seemann]

Currently all of our Grafana dashboards are available right next to the instrumented code.
They are:

• swarm
• eventbus
• resource-manager
• identify

We need to make it as easy as possible for our users to import all the dashboards. At the moment, we define 3 dashboards (swarm, rcmgr, eventbus), but with #1356, we'll soon have ~10 dashboards.

One option to simplify things a bit would be to create a single dashboards directory in the root directory that holds all dashboards. Users would still have to open every JSON file and copy-paste it into their Grafana instance, but at least they don't have to walk our directory tree.

Another option would be to evaluate the grafana.com import option:

Apparently you can import a dashboard by URL or by ID. Unfortunately, a URL pointing to a GitHub repository doesn't work, it has to be a grafana.com URL. Maybe there's an easy way to publish our dashboards on grafana.com via a GitHub Action when we cut a release?

[MarcoPolo]

We should also have a README in that directory.

[MarcoPolo]

This is done with all dashboards in a single folder. We can't have a single one-click to import all the dashboards with what Grafana provides.