Enable golint & gofmt, resolve issues (#214)

* Add golint to linters and resolve issues.

I decided to remove constructors for some of the propagation types
because the constructors can be reduced to either using the zero value
or a single, non optional member.

* Enable gofmt and commit fixes

api/metric/api_test.go

@@ -661,6 +661,7 @@ func TestObserver(t *testing.T) {
 }
 
 func checkBatches(t *testing.T, ctx context.Context, labels LabelSet, meter *mockMeter, descriptor *Descriptor) {
+	t.Helper()
 	if len(meter.measurementBatches) != 3 {
 		t.Errorf("Expected 3 recorded measurement batches, got %d", len(meter.measurementBatches))
 	}

api/trace/always_sampler.go

@@ -47,6 +47,6 @@ func (as alwaysSampleSampler) Description() string {
 
 var _ Sampler = alwaysSampleSampler{}
 
-func AlwaysSampleSampler() alwaysSampleSampler {
+func AlwaysSampleSampler() Sampler {
 	return alwaysSampleSampler{}
 }

api/trace/never_sampler.go

@@ -47,6 +47,6 @@ func (ns neverSampleSampler) Description() string {
 
 var _ Sampler = neverSampleSampler{}
 
-func NeverSampleSampler() neverSampleSampler {
+func NeverSampleSampler() Sampler {
 	return neverSampleSampler{}
 }