Add some documentation for statsd

Change-Id: Ie93681bec4c12bfda5c3c31d34e3cb02176e252d
Hide the token in the worker logs.

Token will no longer be stored as plain text. Will be replaced
with the string '*****' instead.

Change-Id: I14216fd77aef5adcad880c0bb484e69f7c277fbb