Add distributed tracing with LabKit

Makefile

README.md

internal/gitaly/gitaly.go

internal/objectstore/object.go

internal/sendurl/sendurl.go

internal/upstream/roundtripper/roundtripper.go

internal/upstream/routes.go

internal/zipartifacts/open_archive.go

main.go

...golang/protobuf/protoc-gen-go/descriptor/descriptor.pb.go

...golang/protobuf/protoc-gen-go/descriptor/descriptor.proto

...c-ecosystem/go-grpc-middleware/tracing/opentracing/DOC.md

...cosystem/go-grpc-middleware/tracing/opentracing/README.md

...rpc-middleware/tracing/opentracing/client_interceptors.go

...c-ecosystem/go-grpc-middleware/tracing/opentracing/doc.go

...stem/go-grpc-middleware/tracing/opentracing/id_extract.go

...system/go-grpc-middleware/tracing/opentracing/metadata.go

...osystem/go-grpc-middleware/tracing/opentracing/options.go

...rpc-middleware/tracing/opentracing/server_interceptors.go

vendor/github.com/lightstep/lightstep-tracer-go/CHANGELOG.md