Added three new metrics:
- proxyIdleCount counts the number of times a proxy polls and receives
no snowflakes
- clientDeniedCount counts the number of times a client requested a
snowflake but none were available
- clientProxyMatchCount counts the number of times a client successfully
received a snowflake
Previously the metrics log file was hardcoded and the broker wasn't
behaving properly if it was unable to open the file for logging.
Added a commandline option to specify the logfile that defaults to
Stdout.
Fixed up some documentation and log output formatting
