This uses a debug envvar to optionally disable filter logging rate
limits by setting the environment variable
TS_DEBUG_FILTER_RATE_LIMIT_LOGS to "all", and if it matches,
the code will effectively disable the limits on the log rate by
setting the limit to 1 millisecond. This should make sure that all
filter logs will be captured.
Signed-off-by: Christine Dodrill <xe@tailscale.com>