[postgres] Allow 'host' config value be UNIX socket path (#2734)

The PostgreSQL check has been changed to try to connect to the
PostgreSQL database via a UNIX socket if the hostname provided in the
configuration begins with a forward-slash ('/').

This behaviour is similar to the 'psql' command-line utility. From
psql(1):

  --host=hostname
      Specifies the host name of the machine on which the server is
      running. If the value begins with a slash, it is used as the
      directory for the Unix-domain socket.

With the difference being that we interpret it as an exact path, whereas
psql interprets it as a directory to look for a socket with a
particularly formatted name.

checks.d/postgres.py

@@ -602,6 +602,11 @@ def get_connection(self, key, host, port, user, password, dbname, ssl, connect_f
                 elif port != '':
                     connection = connect_fct(host=host, port=port, user=user,
                         password=password, database=dbname, ssl=ssl)
+                elif host.startswith('/'):
+                    # If the hostname starts with /, it's probably a path
+                    # to a UNIX socket. This is similar behaviour to psql
+                    connection = pg.connect(unix_sock=host, user=user,
+                        password=password, database=dbname)
                 else:
                     connection = connect_fct(host=host, user=user, password=password,
                         database=dbname, ssl=ssl)

conf.d/postgres.yaml.example

@@ -12,6 +12,10 @@ instances:
 #      - optional_tag1
 #      - optional_tag2
 
+# Connect using a UNIX socket (host must begin with a /)
+#  - host: /run/postgresql/.s.PGSQL.5433
+#    dbname: db_name
+
 # Track per-relation (table) metrics
 # The list of relations/tables must be specified here.
 # Each relation generates many metrics (10 + 10 per index)