Add ai3/go-common/tracing and deps

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

vendor/go.opencensus.io/trace/export.go

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

vendor/go.opencensus.io/trace/propagation/propagation.go

vendor/go.opencensus.io/trace/sampling.go

vendor/go.opencensus.io/trace/spanbucket.go

vendor/go.opencensus.io/trace/spanstore.go

vendor/go.opencensus.io/trace/status_codes.go

vendor/go.opencensus.io/trace/trace.go

vendor/go.opencensus.io/trace/trace_go11.go

vendor/go.opencensus.io/trace/trace_nongo11.go

vendor/go.opencensus.io/trace/tracestate/tracestate.go

vendor/vendor.json