Prometheus receiver: Move job and instance labels into resource attributes

[scheler]

Is your feature request related to a problem? Please describe.
Prometheus adds the labels job and instance to identify the target scraped - this maps to the Resource in opentelemetry against which the metrics are associated with. Looks like the Resource is currently empty, we could populate these two labels as attributes under Resource.

Describe the solution you'd like
Move (or copy) the two labels job and instance as attributes under Resource.

Describe alternatives you've considered
None

Additional context
None

[odeke-em]

@bjrara resolved this in PR #2897, but didn't use Fixes #2499 in the commit message so it didn't auto-close this issue. Please help close this issue @alolita @tigrannajaryan @rakyll.

[rakyll]

Duplicate of open-telemetry/prometheus-interoperability-spec#37.

#2897 is not solving this issue.