Move pulp metrics-exporter to operate-first

[harshad16]

Is your feature request related to a problem? Please describe.

We have earlier for development kept the pulp metrics-exporter bits in thoth-application and deployed on staging cluster.
However as pulp is service hosted by operate-first, it would be good to have the pulp-metrics-exporter as well included there.

Acceptance criteria

• Move pulp-metrics-exporter to https://github.com/operate-first/apps/tree/master/pulp
• Remove its deployment from stage

[harshad16]

/triage accepted
/lifecycle frozen
/priority important-longterm
/sig devsecops