[agent] change the default journald directory (#1110)

* [agent] change the default journald directory

* update agent metric expectations

* use an empty string as default value

* Revert "use an empty string as default value"

This reverts commit b2aa040.

.chloggen/changedefaultjournalddirectory.yaml

@@ -0,0 +1,12 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+# The name of the component, or a single word describing the area of concern, (e.g. agent, clusterReceiver, gateway, operator, chart, other)
+component: agent
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Change the default directory of the journald receiver
+# One or more tracking issues related to the change
+issues: [1110]
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: