Adds ccp_settings_gauge to OTel metrics

This commit adds the folowing metrics:
- ccp_settings_gauge_checkpoint_timeout
- ccp_settings_gauge_checkpoint_completion_target
- ccp_settings_gauge_shared_buffers

These metrics have been supported for PGO on
postgres_exporter. They are now supported for
PGO on OpenTelemetry.

Issue: PGO-2473

Author: tony-landreth