Add /rules/raw row to Observatorium API dashboard + refactor

[jessicalins]

This PR refactors observatorium-api.libsonnet to reuse common parts and ideally makes it easier to add new rows/panels to the Observatorium API grafana dashboard.

I've added also a new row for /rules/raw - so it makes easier to debug once alerts fire for our Rules path (for example when following this runbook)

Testing dashboard here
EDIT: Updated testing dashboard with error codes here

Availability is low for /rules/raw read since we don't have traffic reading this endpoint. I think next step would be re adapt obsctl-reloader to constant 'read' this endpoint and generate constant traffic there

[douglascamata]

From looking at the test dashboard, in the errors chart we're using the caption "Errors" for all the series returned by the query sum(rate(http_requests_total{job="observatorium-observatorium-api",handler="rules-raw",code=~"5.."}[5m])) / sum(rate(http_requests_total{job="observatorium-thanos-query",handler="rules-raw"}[5m])).

I think would be very useful to include the code label in the caption to help out with debugging. What do you think?

This is a feedback that I have regarding many of our dashboards and I plan to start similar discussions/changes everywhere that it's applicable.

[jessicalins]

@douglascamata that's a good idea! let me try adding this in this PR for this dashboard. In case this applies to other dashboards, maybe we could add a ticket/issue so that this is not forgotten?

[douglascamata]

@jessicalins when I get some time I will figure out where this hardcoded "Error" label come from in the other dashboards (http and grpc errors too) and create an issue(s) in the repo(s) + a ticket on our side. 👍

[douglascamata]

FYI I created an issue for the other dashboards widgets: #348

[douglascamata]

I think the query isn't returning exactly what we expect now.

[douglascamata]

LGTM 👍