Move ccp_stat_database metrics/query to 5 second interval receiver.

Move ccp_stat_user_tables metrics/query to 5 minute interval receiver.
Add comment about null values to ccp_replication_slots metrics/query.

Author: dsessler7

internal/collector/eq_pg16_metrics.yaml renamed to internal/collector/eq_pg16_fast_metrics.yaml

@@ -4,6 +4,9 @@
 # https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/-/receiver/sqlqueryreceiver#metrics-queries
 # https://github.com/CrunchyData/pgmonitor/blob/development/sql_exporter/common/crunchy_global_collector.yml
 
+# NOTE: Some of the columns below can return NULL values, for which sqlqueryreceiver will warn.
+# https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/receiver/sqlqueryreceiver#null-values
+# Those columns are retained_bytes, database, and conflicting and we avoid NULL by using COALESCE.
   - sql: >
       SELECT
           s.slot_name