Introduce packaging to Docker Mixin

[malcolmholmes]

A proposal for a packaging format for observability packages that is back-compatible with
Jsonnet Mixins.

[malcolmholmes]

to test, render this:

• First, create a file, m.jsonnet that makes the hidden elements in a mixin visible:

local mixin = import 'mixin.libsonnet';
{
  grafanaDashboards: {
    [key]: mixin.grafanaDashboards[key]
    for key in std.objectFields(mixin.grafanaDashboards)
  }
}

• Next, execute: jsonnet -J vendor m.jsonnet
• This will give you the json output from the mixin.
• Alternatively, execute: jsonnet -J vendor polly.libsonnet to get the package contents.

[v-zhuravlev]

Sometimes variables depend on config (instead of hardcoded job=~"$job" it could be dynamic label selector). Does this packaging provide a way to provide custom labelsSelector from config?

[malcolmholmes]

This is entirely up to us to define, so I'm sure it can!

[v-zhuravlev]

Quite common that query selectors depend on config (instead of hardcoded job=~"$job" it could be user provided label selector). Does this packaging provide a way to provide custom host_matcher from config?

[v-zhuravlev]

I think queries are better packed as targets (query + legend)

[Duologic]

I'd love to see a more abstract description of the format, it's hard to untangle the format from the data in this PR.