Allow custom display name format for spans (#45)

[stevencl1013]

This allows users to, when creating the exporter, use a custom DisplayNameFormatter function to specify how to create the display name from span data. Default format remains the same. Addressing issue #45

[ymotongpoo]

@stevencl1013 Can you run make precommit? It's failing as the following:

=== RUN   TestExporter_Timeout
    TestExporter_Timeout: cloudtrace_test.go:157: err.Error() = "rpc error: code = Unknown desc = context deadline exceeded"; want "rpc error: code = DeadlineExceeded desc = context deadline exceeded"
--- FAIL: TestExporter_Timeout (0.00s)
FAIL
FAIL	github.com/GoogleCloudPlatform/opentelemetry-operations-go/exporter/trace	0.029s
FAIL
make: *** [Makefile:85: test-386] Error 1

[stevencl1013]

@ymotongpoo When I ran make precommit on my machine, all the tests passed. I think there might be an issue with the timeout test, because it did get a context deadline exceeded error, but failed because it got rpc error: code = Unknown instead of rpc error: code = DeadlineExceeded. I've experienced this issue occasionally on my machine as well in the past.

[qqqstuv]

LGTM.

Does the other languages also support custom span name? We should coordinate with them to lepe the feature parity among languages

[nilebox]

LGTM, please address the comments.

[nilebox]

Please rename this variable to displayName - Span has the Name field which will have completely different value (see below).
Also, the initialization with s.Name value seems redundant since it will always be overwritten below.

[nilebox]

I would actually suggest to extract this entire code block into generateDisplayName(s, format) to make the code cleaner / easier to read.

[james-bebbington]

LGTM

[james-bebbington]

It's always a little easier to read code if you minimize nesting imo:

Suggested change

	if format == nil {
	switch s.SpanKind {
	// TODO(ymotongpoo): add cases for "Send" and "Recv".
	default:
	displayName = fmt.Sprintf("Span.%s-%s", s.SpanKind, s.Name)
	}
	} else {
	displayName = format(s)
	}
	if format != nil {
	return format(s)
	}
	switch s.SpanKind {
	// TODO(ymotongpoo): add cases for "Send" and "Recv".
	default:
	return fmt.Sprintf("Span.%s-%s", s.SpanKind, s.Name)
	}

[james-bebbington]

Sleeping for a block amount of time in tests in never ideal. To make this signficiantly more robust, and faster, a better alternative to this is to do something like

assert.Eventually(t, function() {
    ...
}, 500 * time.Millisecond, 2 * time.Millisecond)

I wouldn't bother changing this now though. After you add the NewPipeline method that returns a flush function (see the Jaeger exporter), you can avoid the sleep here entirely.

[stevencl1013]

ok, sounds good. I'll leave this for now, and when I create the NewPipeline method, I plan to update this test and others to use the pipeline and flush function as appropriate.