Initial import of lightstepreceiver

[carlosalberto]

Initial import of our lightstepreceiver, which will allow us to ingest data from legacy tracers. Overall:

• Defaults to sending data to localhost:443. Maybe reconsider?
• Support for http/proto, with upcoming support for grpc (and maybe thrift).
• No initial OBS report for now (i.e. no internal metrics reporting traces count and related)
• Semantic conventions mapping is only done for errors, i.e. semconv is kept from OpenTracing.
• Other limitations or potential improvements are listed in DESIGN.md

[nslaughter]

1. Running gofmt -w -s . and goimports -w . in the receiver directory will correct some formatting. Running those commands with path set to ./.. could change generated files and that's generally discouraged.
2. The substantial parts of missing coverage are in trace_receiver.go, so adding something to the tests in there would be great even though those look like well known methods. Some coverage there could be especially good for running tests with -race.

[nslaughter]

Running gofmt -w -s . and goimports -w . in the receiver directory will correct some formatting. Running those commands with path set to ./.. could change generated files and that's generally discouraged.

The substantial parts of missing coverage are in trace_receiver.go, so adding something to the tests in there would be great even though those look like well known methods. I think it's a worthwhile enhancement even if you're going for a lift and shift port. Afterward you can run the tests with go test ./.. -race and use goleak to verify the code doesn't leak goroutines. I checked this out to be sure although I think it would be covered with lifecycle tests.. if this were in OTel repos.

[nslaughter]

LGTM
Thanks for doing this, Carlos.

[jmacd]

By the way, I've updated some dependencies and fixed one of the leak-tests.
There is now a proto/ directory containing the lightstep protos, the google protos we depend on, and a script to re-run protoc.

[jmacd]

Also having re-run a more-recent mdatagen on this component, there are new lifecycle tests and somewhat better coverage (now around 80%).

[nslaughter]

Merging this port of Lightstep receiver based on unit tests and manual reviews.