Upgrade ai3/go-common (tracing)

go.mod

go.sum

vendor/contrib.go.opencensus.io/exporter/zipkin/.travis.yml

vendor/contrib.go.opencensus.io/exporter/zipkin/Makefile

vendor/contrib.go.opencensus.io/exporter/zipkin/README.md

vendor/contrib.go.opencensus.io/exporter/zipkin/go.mod

vendor/contrib.go.opencensus.io/exporter/zipkin/go.sum

vendor/contrib.go.opencensus.io/exporter/zipkin/zipkin.go

vendor/git.autistici.org/ai3/go-common/go.mod

vendor/git.autistici.org/ai3/go-common/go.sum

vendor/git.autistici.org/ai3/go-common/tracing/tracing.go

vendor/github.com/felixge/httpsnoop/.gitignore

vendor/github.com/felixge/httpsnoop/.travis.yml

vendor/github.com/felixge/httpsnoop/LICENSE.txt

vendor/github.com/felixge/httpsnoop/Makefile

vendor/github.com/felixge/httpsnoop/README.md

vendor/github.com/felixge/httpsnoop/capture_metrics.go

vendor/github.com/felixge/httpsnoop/docs.go

vendor/github.com/felixge/httpsnoop/go.mod

...r/github.com/felixge/httpsnoop/wrap_generated_gteq_1.8.go