.etl file extension is used by standart windows performance monitoring

It's binary file
Run :tracerpt.exe your.etl from command line and your file will be processed to readable csv file and text summary.
or tracert.exe /? to view help information:

>tracerpt /?
Microsoft TraceRpt.Exe (5.1.2600.2180)
Microsoft Corporation. All rights reserved.

Tracerpt processes binary Event Trace Session log files or real-time streams from instrumented Event Trace providers and creates a report or 
a text (CSV) file describing the events generated.

Usage:
tracerpt { <filename [filename ...]> | -rt <session_name [session_name ...]> } [options]

Parameters:
  <filename [filename ...]>     Event Trace log file to process.

Options:
  -?                            Displays context sensitive help.
  -o [filename]                 Text (CSV) output file. Default is dumpfile.csv.
  -summary [filename]           Summary report text file (CSV) file. Default is summary.txt.
  -report [filename]            Text output report file. Default is workload.txt.
  -rt <session_name [session_name ...]>   Real-time Event Trace Session data source.
  -config <filename>            Settings file containing command options.
  -y                            Answer yes to all questions without prompting.

Examples:
  tracerpt logfile1.etl logfile2.etl -o -report
  tracerpt logfile.etl -o logdmp.csv -summary logdmp.txt -report logrpt.txt
  tracerpt -rt EVENT_SESSION_1 EVENT_SESSION_2 -o logfile.csv

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