Add support for gzip encoded payload in OTLP/HTTP receiver

[code-haven]

Description:
Fixing a bug - OTLP/HTTP receiver does not support gzip. Fixed this by wrapping the HTTP handler generated by grpc-gateway to decompress prior to processing the request, similar to how it's done in

opentelemetry-collector/receiver/zipkinreceiver/trace_receiver.go

Line 208 in c4a3829

func processBodyIfNecessary(req *http.Request) io.Reader {

Link to tracking Issue:
#1344

Testing:
Modified unit tests to cover cases where request body is compressed via gzip/zlib.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Abhinav Ittekot (60e9ecd)

[codecov]

Codecov Report

Merging #1581 into master will increase coverage by 0.00%.
The diff coverage is 92.40%.

@@           Coverage Diff           @@
##           master    #1581   +/-   ##
=======================================
  Coverage   91.67%   91.67%           
=======================================
  Files         261      262    +1     
  Lines       18540    18617   +77     
=======================================
+ Hits        16997    17068   +71     
- Misses       1113     1117    +4     
- Partials      430      432    +2     

Impacted Files	Coverage Δ
receiver/otlpreceiver/otlphttp.go	78.57% <76.92%> (-21.43%)	⬇️
config/confighttp/confighttp.go	100.00% <100.00%> (ø)
internal/middleware/compression.go	100.00% <100.00%> (ø)
receiver/otlpreceiver/otlp.go	95.00% <100.00%> (+0.19%)	⬆️
translator/internaldata/resource_to_oc.go	92.59% <0.00%> (ø)

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[tigrannajaryan]

@code-haven thank you for the PR, I left a couple comments/questions.

[bogdandrutu]

@code-haven there are few unresolved comments, please ping when everything is resolved :)

[bogdandrutu]

Please rebase :)

[code-haven]

@bogdandrutu @tigrannajaryan @jrcamp So I found a way to identify within the marshallers itself if the request has gzipped content by replicating what gzlib library does interally for validation.
I think this may be the right approach to solve this without resorting to the hacky approach of replacing the request body inside a middleware. Response compression can be handled via a middleware like https://github.com/nytimes/gziphandler.
I sincerely apologize for making so many changes in the PR :(. I'm converting this to a draft PR & also limiting scope to gzip only (as per the original issue). I should be ready with the final set of changes by tomorrow.

[code-haven]

Please rebase :)

@bogdandrutu I've decided to roll back the changes until this comment as I realize I'm complicating things by making changes to this PR without discussion. Sorry for all the confusion :(

Wanted to discuss if something like this would work instead of replacing the r.Body by peaking header bytes as defined in rfc1952? This would be implemented in xProtobufMarshaler

// NewDecoder returns a Decoder which reads proto stream from "reader".
func (marshaller *xProtobufMarshaler) NewDecoder(reader io.Reader) runtime.Decoder {
	return runtime.DecoderFunc(func(value interface{}) error {
		reader, gzipped, err := isGzip(reader)
		if err != nil {
			return err
		}
		if gzipped {
			gzReader, err := gzip.NewReader(reader)
			if err != nil {
				return err
			}
			reader = gzReader
			defer gzReader.Close()
		}
		buffer, err := ioutil.ReadAll(reader)
		if err != nil {
			return err
		}
		return marshaller.Unmarshal(buffer, value)
	})
}


func isGzip(input io.Reader) (io.Reader, bool, error) {
	const (
		gzipID1     = 0x1f
		gzipID2     = 0x8b
		gzipDeflate = 8
		peakLength  = 3
	)
        // standard gzip header bytes as defined in 
	reader := bufio.NewReader(input)
	headerBytes, err := reader.Peek(peakLength)
	if err != nil {
		return reader, false, err
	}
	isGzip := headerBytes[0] == gzipID1 && headerBytes[1] == gzipID2 && headerBytes[2] == gzipDeflate
	return reader, isGzip, nil
}

[tigrannajaryan]

@code-haven please fix the build.

[tigrannajaryan]

LGTM, with minor comment.

[bogdandrutu]

I would use if instead here.

[tigrannajaryan]

@bogdandrutu do you have any other concerns or I can merge?