r/fluent-bit: Support drop-in configuration files

Fluent Bit supports including configuration fragments from other files
using its `includes` option.  Adding a glob pattern to the default
configuration will allow other roles to supply additional configuration
by creating files in the `/etc/fluent-bit/include` directory.  This
makes composition of configuration significantly easier.

Unfortunately, `fluent-bit` has a quirk in that there must exist at
least one file matching the glob pattern, or it will fail to start.  To
work around this, we must supply an empty fragment.

roles/fluent-bit/defaults/main.yml

@@ -1,4 +1,6 @@
 fluent_bit_config:
+  includes:
+  - /etc/fluent-bit/include/*.yml
   service: '{{ fluent_bit_config_service }}'
   parsers: '{{ fluent_bit_parsers }}'
   multiline_parsers: '{{ fluent_bit_multiline_parsers }}'

roles/fluent-bit/tasks/main.yml

@@ -5,6 +5,29 @@
   tags:
   - install
 
+- name: ensure fluent-bit config include directory exists
+  file:
+    path: /etc/fluent-bit/include
+    owner: root
+    group: root
+    mode: u=rwx,go=
+    state: directory
+  tags:
+  - config
+# fluent-bit will fail to start if a glob pattern listed in `includes` does not
+# match any files.  To allow roles to drop additiona configuration files in the
+# include directory without modifying the main configuration file, we need to
+# have an empty file in the include directory all the time.
+- name: ensure fluent-bit empty include file exists
+  copy:
+    dest: /etc/fluent-bit/include/_.yml
+    content: ''
+    owner: root
+    group: root
+    mode: u=rwx,go=
+  tags:
+  - config
+
 - name: ensure fluent-bit is configured
   copy:
     dest: /etc/fluent-bit/fluent-bit.yml