Implement OTLP/HTTP X-Protobuf Receiver

[kirbyquerby]

Description: This change adds application/x-protobuf support to the existing
grpc-gateway mux in the OTLP receiver.

(This change is a duplicate of #982 because CircleCI wasn't working.)

Protocol spec: https://github.com/open-telemetry/oteps/blob/master/text/0099-otlp-http.md

What currently works:

• The receiver will correctly process HTTP requests with the
  Content-Type application/x-protobuf.
• The receiver will respond with Content-Type application/x-protobuf.
• The receiver will respond with status code HTTP 200 OK on success.
• The receiver can handle gzip-encoded requests (Content-Encoding
  gzip) (http.Server natively supports gzip decoding).

What doesn't work yet:

• The receiver will not gzip-encode responses to requests with
  Accept-Encoding: gzip.

Link to tracking Issue: #881

Testing: A test was added to verify that the receiver accepts application/x-protobuf requests.

Documentation:

[codecov]

Codecov Report

Merging #1021 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1021      +/-   ##
==========================================
+ Coverage   85.61%   85.63%   +0.01%     
==========================================
  Files         189      190       +1     
  Lines       13167    13171       +4     
==========================================
+ Hits        11273    11279       +6     
+ Misses       1441     1440       -1     
+ Partials      453      452       -1     

Impacted Files	Coverage Δ
receiver/otlpreceiver/otlp.go	86.72% <100.00%> (+0.23%)	⬆️
receiver/otlpreceiver/otlphttp.go	100.00% <100.00%> (ø)
translator/internaldata/resource_to_oc.go	76.47% <0.00%> (+2.94%)	⬆️

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[tigrannajaryan]

LGTM. Once OTLP/HTTP exporter is added we will need to add OTLP/HTTP testing to E2E tests.

[tigrannajaryan]

What doesn't work yet:

• The receiver cannot handle gzip-encoded requests (Content-Encoding
  gzip).
• The receiver will not gzip-encode responses to requests with
  Accept-Encoding: gzip.

@kirbyquerby do you plan to submit a separate PR for these? Can you add a github issue so that we don't forget?

[kirbyquerby]

@tigrannajaryan for the first bullet point I was mistaken: Go's http server can already receive gzip-encoded requests (but still will not gzip responses). I am planning on submitting a PR for gzipped responses soon.