fluent-bit: Add hostname field to all records

Messages from sources other than the systemd journal do not have a
`hostname` field by default.  This could make filtering logs difficult
if there are multiple servers that host the same application.  Thus, we
need to inject the host name statically into every record, to ensure
they can be correctly traced to their source machine.

group_vars/all.yml

@@ -143,6 +143,10 @@ dnf_automatic_schedule: >-
   }} *-*-* 04:00:00 America/Chicago
 
 fluent_bit_default_filters:
+- name: record_modifier
+  match: '*'
+  record:
+  - hostname ${HOSTNAME}
 # Avoid log amplification from logging the result of sending logs!
 - name: grep
   match: host.fluent-bit.service