[receiver/postgresql] Add bgwriter Metrics

[schmikei]

Is your feature request related to a problem? Please describe.
Adding more metrics initially spun up from #12338

These are the bgwriter metrics related to the background process introduced in postgres 8.0 that occasionally flushes modified pages out of the shared buffer pool.

More information on the source of these metrics can be found here.

Planning on putting this behind the feature gate: receiver.postgresql.emitMetricsWithResourceAttributes

Describe the solution you'd like
Metrics proposing to add:

postgresql.bgwriter.buffers.allocated
postgresql.bgwriter.buffers.writes
postgresql.bgwriter.checkpoint.count
postgresql.bgwriter.duration
postgresql.bgwriter.maxwritten.count

Describe alternatives you've considered

Additional context

[schmikei]

I will be working on this!