Add ai3/go-common/tracing and deps

vendor/github.com/openzipkin/zipkin-go/reporter/reporter.go

vendor/github.com/openzipkin/zipkin-go/sample.go

vendor/github.com/openzipkin/zipkin-go/span.go

...or/github.com/openzipkin/zipkin-go/span_implementation.go

vendor/github.com/openzipkin/zipkin-go/span_options.go

vendor/github.com/openzipkin/zipkin-go/tags.go

vendor/github.com/openzipkin/zipkin-go/tracer.go

vendor/github.com/openzipkin/zipkin-go/tracer_options.go

vendor/go.opencensus.io/AUTHORS

vendor/go.opencensus.io/CONTRIBUTING.md

vendor/go.opencensus.io/Gopkg.lock

vendor/go.opencensus.io/Gopkg.toml

vendor/go.opencensus.io/LICENSE

vendor/go.opencensus.io/README.md

vendor/go.opencensus.io/appveyor.yml

vendor/go.opencensus.io/exemplar/exemplar.go

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

vendor/go.opencensus.io/go.mod

vendor/go.opencensus.io/go.sum

vendor/go.opencensus.io/internal/internal.go