Add backend generated metric postgresql.connections_by_process metadata (#20052)

* testing a branch

* add postgresql.connections_by_process metadata

* Delete allen-test-file.txt

Author: azhou-datadog

postgres/metadata.csv

@@ -45,7 +45,8 @@ postgresql.conflicts.deadlock,count,,query,,"Number of queries in this database
 postgresql.conflicts.lock,count,,query,,Number of queries in this database that have been canceled due to lock timeouts. This will occur when the walreceiver process tries to apply a change requiring an ACCESS EXCLUSIVE lock while a query on the replica is reading the table. The conflicting query will be killed after waiting up to max_standby_streaming_delay seconds. This metric is tagged with db.,-1,postgres,cfl lock,
 postgresql.conflicts.snapshot,count,,query,,"Number of queries in this database that have been canceled due to old snapshots. Snapshot conflict will occur when a VACUUM is replayed, removing tuples currently read on a standby. This metric is tagged with db.",-1,postgres,cfl snapshot,
 postgresql.conflicts.tablespace,count,,query,,Number of queries in this database that have been canceled due to dropped tablespaces. This will occur when a temp_tablespace is dropped while being used on a standby. This metric is tagged with db.,-1,postgres,cfl tablespace,
-postgresql.connections,gauge,,connection,,"The number of active connections to this database. If DBM is enabled, this metric is tagged with state, app, db and user",0,postgres,conns,
+postgresql.connections,gauge,,connection,,"The number of active connections to this database.",0,postgres,conns,
+postgresql.connections_by_process,gauge,,connection,,"The number of active connections to this database, broken down by process-level attributes. This metric is tagged with state, application, user, and db. (DBM only)",0,postgres,conns,
 postgresql.control.checkpoint_delay,gauge,,second,,The time since the last checkpoint.,0,postgres,control checkpoint,
 postgresql.control.checkpoint_delay_bytes,gauge,,byte,,The amount of WAL bytes written since the last checkpoint.,0,postgres,control checkpoint byte,
 postgresql.control.redo_delay_bytes,gauge,,byte,,The amount of WAL bytes written since the last redo point.,0,postgres,control redo byte,