add aggregated rtp connection stats to osmo-mgw

Add a counter group for aggregated RTP connection statistics.
This group contains RTP counters which aggregate values of the
ephemeral RTP counters maintained per connection (mgcp_conn).

This provides a global overview of RTP processing for each
trunk throughout the lifetime of the osmo-mgw process.

The counters are displayed by 'show mgcp stats' and 'show rate-counters'.

While here, fix a typo in an item of the mgcp_conn_rate_ctr_desc array:
"rtp:octets_rx" was displayed for outbound packes; now says "_tx".

Change-Id: I80d36181600901ae2e0f321dc02b5d54ddc94139
Related: OS#2660
diff --git a/include/osmocom/mgcp/mgcp.h b/include/osmocom/mgcp/mgcp.h
index c8c2cfd..aa5607f 100644
--- a/include/osmocom/mgcp/mgcp.h
+++ b/include/osmocom/mgcp/mgcp.h
@@ -194,9 +194,12 @@
 	int vty_number_endpoints;
 	struct mgcp_endpoint *endpoints;
 
-	/* rate counters */
+	/* Rate counter group which contains stats for processed CRCX commands. */
 	struct rate_ctr_group *mgcp_crcx_ctr_group;
+	/* Rate counter group which contains stats for processed MDCX commands. */
 	struct rate_ctr_group *mgcp_mdcx_ctr_group;
+	/* Rate counter group which aggregates stats of individual RTP connections. */
+	struct rate_ctr_group *all_rtp_conn_stats;
 };
 
 enum mgcp_role {