"{c}L" exists and logs the "Log-Id of the Connection".
I'm not sure of the description to write there, so I just mention it in the doc and hope s.o. will find the words :)
This has been introduced in r992806 and is also part of 2.4.
logged to the error log for this request). Look for the
matching error log line to see what request caused what error.</td></tr>
+ <tr><td><code>%{c}L</code></td>
+ <td></td></tr>
+
<tr><td><code>%m</code></td>
<td>The request method.</td></tr>
