Include RDS Proxy metrics within the RDS namespace.

[vitaliyf]

Example output:

aws_rds_database_connections_sum{account_alias="example-account",account_id="redacted-id-here",dimension_DBInstanceIdentifier="",dimension_DatabaseClass="",dimension_EngineName="",dimension_ProxyName="mysql-example-proxy",dimension_Target="",dimension_TargetGroup="",dimension_TargetRole="",name="global",region="us-west-2"} 5595
aws_rds_database_connections_sum{account_alias="example-account",account_id="redacted-id-here",dimension_DBInstanceIdentifier="",dimension_DatabaseClass="",dimension_EngineName="",dimension_ProxyName="mysql-example-proxy",dimension_Target="",dimension_TargetGroup="default",dimension_TargetRole="READWRITE",name="global",region="us-west-2"} 5595
aws_rds_database_connections_sum{account_alias="example-account",account_id="redacted-id-here",dimension_DBInstanceIdentifier="",dimension_DatabaseClass="",dimension_EngineName="",dimension_ProxyName="mysql-example-proxy",dimension_Target="",dimension_TargetGroup="default",dimension_TargetRole="READ_ONLY",name="global",region="us-west-2"} 0
aws_rds_database_connections_sum{account_alias="example-account",account_id="redacted-id-here",dimension_DBInstanceIdentifier="",dimension_DatabaseClass="",dimension_EngineName="",dimension_ProxyName="mysql-example-proxy",dimension_Target="db:mysql-example-proxy",dimension_TargetGroup="default",dimension_TargetRole="",name="global",region="us-west-2"} 5595
aws_rds_database_connections_sum{account_alias="example-account",account_id="redacted-id-here",dimension_DBInstanceIdentifier="mysql-example-proxy",dimension_DatabaseClass="",dimension_EngineName="",dimension_ProxyName="",dimension_Target="",dimension_TargetGroup="",dimension_TargetRole="",name="arn:aws:rds:us-west-2:redacted-id-here:db:mysql-example-proxy",region="us-west-2"} 6251

Fixes #1460

[fafg]

what need to be improved or what is missing to be merged?