log: add gauge for tree size
log: add gauge for tree size

AlCutter/trillian,gdbelvin/trillian,gdbelvin/trillian,RJPercival/trillian,Martin2112/trillian,google/trillian,Martin2112/trillian,Martin2112/trillian,daviddrysdale/trillian,daviddrysdale/trillian,gdbelvin/trillian,google/trillian,codingllama/trillian,codingllama/trillian,AlCutter/trillian,RJPercival/trillian,daviddrysdale/trillian