Make it possible to filter on arbitrary journald fields

This commit adds a new option, journald_filters, which allows to pass a
list of filters of the form <journald_field>=<value>.

For a list of available journald fields, see:
https://www.freedesktop.org/software/systemd/man/latest/systemd.journal-fields.html

Author: rdunklau

README.rst

@@ -326,6 +326,13 @@ This feature requires latest version of ``python-systemd`` `with namespace suppo
 Require that the logs message matches only against certain _SYSTEMD_UNITs.
 If not set, we allow log events from all units.
 
+``journald_filters`` (default ``[]``)
+
+Journald filters in the form of <journald_field>=<value>. If not set, no filter will be applied.
+Example: SYSLOG_IDENTIFIER=postgres
+
+Refer to https://www.freedesktop.org/software/systemd/man/latest/systemd.journal-fields.html for a list of valid fields.
+
 ``flags`` (default ``LOCAL_ONLY``)
 
 ``"LOCAL_ONLY"`` opens journal on local machine only; ``"RUNTIME_ONLY"`` opens only volatile journal files;

journalpump/journalpump.py

@@ -468,7 +468,8 @@ def get_reader(self, seek_to=None, reinit=False):
 
         for unit_to_match in self.config.get("units_to_match", []):
             self.journald_reader.add_match(_SYSTEMD_UNIT=unit_to_match)
-
+        for journald_filter in self.config.get("journald_filters", []):
+            self.journald_reader.add_match(journald_filter)
         self.initialize_senders()
 
         return self.journald_reader