What is the format of the snrlog

Are you referring to the temporary snr log file in the node's /tmp/snrlog directory?  I guess the answer depends on what you want to do with that data.  It is straightforward to parse the more robust data available via the sysinfo API (formerly sysinfo.json).  Sysinfo shows all of these values (and more) with appropriate identifiers:

"rxcost": 256,
"txcost": 256,
"signal": -32,
"tx_packets": 593,
"tx_retries": 22,
"tx_fail": 0,
"tx_bitrate": 36.1,
"rx_bitrate": 36.1,
"snr": 56,

According to the code, the /tmp/snrlog/xxx... files show the date/time, signal, noise, tx mcs, tx bitrate, rx mcs, rx bitrate, bw adjust 
but that file was intended for internal use rather than as a data source for other apps.