Update descriptions of XMPP stanza and data metrics

Affected metrics:
- xmpp_element_size_in and xmpp_element_out, which replaced
  the 'element' and 'size' metrics.
- c2s_auth_failed, s2s_auth_failed, component_auth_failed

Other changes in the docs:
- Dropped distintion between host-type-specific and global metrics,
  because it was difficult to group metrics by functionality,
  and most metrics were decribed with labels anyway.
- Merged descriptions of data metrics (tcp_data_in, tcp_data_out,
  tls_data_in, tls_data_out) for improved consistency and reduced verbosity.
- Added a new column with labels where labels differ between rows.

Author: chrzaszcz

doc/operation-and-maintenance/Logging-&-monitoring.md

@@ -108,8 +108,8 @@ For time-based metrics, you can choose to display multiple calculated values for
 
 ```
 Session count:                   <prefix>.global.sm_total_sessions.count
-Outgoing XMPP messages:          <prefix>.<domain>.c2s_element_out.message_count.one
-Incoming XMPP messages:          <prefix>.<domain>.c2s_element_in.message_count.one
+Outgoing XMPP messages:          <prefix>.<domain>.xmpp_element_out.c2s.message_count.one
+Incoming XMPP messages:          <prefix>.<domain>.xmpp_element_in.c2s.message_count.one
 Successful logins:               <prefix>.<domain>.sm_session.logins.one
 Logouts:                         <prefix>.<domain>.sm_session.logouts.one
 Authorization time:              <prefix>.<domain>.backends.auth.authorize.<value-type>