OpenCensus Go 0.19.0

2b5032d Add metric package containing Gauge support
2f39cd4 Update internal import. (#1014)
6e8ca2c Guard bundler from buffering too many messages in memory (#1011)
950a67f Fix data race when registering views multiple times
25040cf Return an error if negative bucket bounds are provided
df705ed Upgrade dependencies; resolve test issue with Prom exporter (#1000)
8b95f1c Add metric data model and export support (#977)
9f101dc Improve the accuracy of the response size in ochttp (#997)
aab39bd Make our user-agent consistent with other user-agents. (#991)
8b019f3 Remove dead code in Prometheus exporter. (#990)
aeef0d7 Fix prometheus exporter when tags are missing. (#989)
fa1e35e Keep pointer receiver name consistency for stats.Measure types (#982)
296c89c Revert "Add metric API (#975)" (#981)
a91cf18 Add metric API (#975)
91a0276 Update README link to database/sql wrapper (#972)
94fd1d0 exporter/prometheus: provide ConstLabels (#962)
4f7fcb4 Add resource package (#878)
96e75b8 Bump version to 0.19.0 (#955)