Merge `master` into `develop`

* Update protobuf file to latest.

f13a86b4b013d2ae07e78c7c27359d0a158be118

* Log gRPC version used.

* Replace usage of deprecated grpc.WithDialer. (#942)

* Prevent debug messages unless debug level set. (#943)

* Include request_headers_map from connect reply in RecordSpan metadata. (#940)

* Fix code strings to match spec. (#945)

* Add parent.* intrinsics from transaction to root span. (#946)

* Update span GUID propagation  (#944)

* Pull in new cross agent tests (not yet passing)

* Code changes for span GUID creation

* Get tests passing

* t.Helper() was added in Go 1.9, so it can't be used here. :(

* PR feedback

* Update commit of cross-agent tests now that PR is merged

* Never use the default transport.

* When a timeout error occurs save instead of drop data.

* Refactor trace_observer.go

* Move defer statement to follow conventions

* Refactor grpc.Dial logic. (#949)

* Enable GitHub Actions for CI testing.

* Add changelog entry.

* Update version for v3.7.0 (#956)

* Update version for v3.7.0

* Fix headings

* Modify Serverless to use a named pipe for telemetry when present

Serverless telemetry is normally sent to stdout. This is an alternate
path designed for use with an externally managed named pipe at a
fixed location.

* Changelog entry

* Formatting

* Fix failing go1.9 tests.

* Update CHANGELOG.md

Co-authored-by: Rey Abolofia <purple4reina@gmail.com>

* Address PR feedback

Co-authored-by: Rey Abolofia <rabolofia@newrelic.com>
Co-authored-by: Elizabeth Heinlein <eheinlein@newrelic.com>
Co-authored-by: Elizabeth Heinlein <52758187+newrelic-eheinlein@users.noreply.github.com>
Co-authored-by: Rey Abolofia <purple4reina@gmail.com>

Add Go version 1.15 to tests

Updating version #