metrics batch is too large

[bsimpson63]

From Thing service logs:

Mar 22 17:08:33 thing-0c5b40c8692e8e5e4 [thing] 15496:17713791717421479062:baseplate.metrics:ERROR:baseplate metrics batch too large: flush more often                 or reduce amount done in one request, length 933145, [Errno 90] Message too long#015

[spladug]

c6b0539 w/ discussion in #255