Expands into the time it took for the handler to complete the
response, expressed as seconds with 6 decimals precision.
+*%{*'HEADER'*}p*::
+
+ Expands into the HTTP response header named by 'HEADER'. If
+ the specified header does not exist in the request, *%p*
+ expands into a dash.
+
+*%{*'HEADER'*}P*::
+
+ Similar to *%p*, except that 'HEADER' is prepended with
+ `X-Ash-`, for simple convenience.
+
In any expanded field, any "unsafe" characters are escaped. Currently,
this means that double-quotes and backslashes are prepended with a
backslash, newlines and tabs are expressed as, respectively, `\n` and