Collecting GSSAPI statistics

[CHERTS]

Is your feature request related to a problem? Please describe.
It is necessary to collect the simplest statistics about GSSAPI:

• Number of encrypted DB connections
• Number of GSSAPI authenticated DB connections
• Number of DB connections with delegated GSSAPI credentials

Describe the solution you'd like
Implement new postgres/gssapi collector

Number of encrypted DB connections:

SELECT COUNT(*) FROM pg_stat_gssapi 
JOIN pg_stat_activity ON pg_stat_gssapi.pid = pg_stat_activity.pid
WHERE encrypted = 't' AND datname IS NOT NULL;

Number of GSSAPI authenticated DB connections:

SELECT COUNT(*) FROM pg_stat_gssapi 
JOIN pg_stat_activity ON pg_stat_gssapi.pid = pg_stat_activity.pid 
WHERE gss_authenticated = 't' AND datname IS NOT NULL;

Number of DB connections with delegated GSSAPI credentials:

SELECT COUNT(*) FROM pg_stat_gssapi 
JOIN pg_stat_activity ON pg_stat_gssapi.pid = pg_stat_activity.pid 
WHERE credentials_delegated = 't' AND datname IS NOT NULL;