Formats a log message.

The following formatting directives are available:

• %% - Percentage sign
• %a - Remote IP-address
• %b - Size of response in bytes. Outputs "-" when no bytes are sent.
• %B - Size of response in bytes. Outputs "0" when no bytes are sent.
• %{VARNAME}C - Cookie sent by the client
• %D - Time taken to serve the request (microseconds)
• %{VARNAME}e - Environment variable contents
• %h - Remote hostname
• %{VARNAME}i - Header line sent by the client
• %l - Remote logname
• %m - Request method
• %M - Time taken to serve the request (milliseconds)
• %{VARNAME}o - Header line sent by the server
• %P - The process ID that serviced the request
• %q - Query string (prepended with "?" or empty string)
• %r - First line of HTTP request
• %>s - Response status code
• %t - Time the request was received in the format [10/Jan/2015:14:46:18 +0100]
• %T - Time taken to serve the request (full seconds)
• %{UNIT}T - Time taken to serve the request in the given UNIT
• %u - Remote user
• %U - URL path requested (without query string)
• %v - Server name
• %V - Server name (canonical)

Note for %b and %B: To determine the size of the response the "Content-Length" will be inspected and, if available, returned unverified. If the header is not present the response body will be inspected using byte_size/1.

Note for %h: The hostname will always be the ip of the client (same as %a).

Note for %l: Always a dash ("-").

Note for %T: Rounding happens, so "0.6 seconds" will be reported as "1 second".

Note for %{UNIT}T: Available units are s for seconds (same as %T), ms for milliseconds (same as M) and us for microseconds (same as %D).

Note for %V: Alias for %v.