[chore][exporter/otlp] Assert interface implementation in tests (#13442)

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#### Description

Minor documentation update to clarify the purpose of a few test structs,
following a recommendation here:
#13424 (comment).

Author: evan-bradley

exporter/otlpexporter/otlp_test.go

@@ -64,6 +64,8 @@ func (r *mockReceiver) setExportError(err error) {
 	r.exportError = err
 }
 
+var _ ptraceotlp.GRPCServer = &mockTracesReceiver{}
+
 type mockTracesReceiver struct {
 	ptraceotlp.UnimplementedGRPCServer
 	mockReceiver
@@ -128,6 +130,8 @@ func otlpTracesReceiverOnGRPCServer(ln net.Listener, useTLS bool) (*mockTracesRe
 	return rcv, nil
 }
 
+var _ plogotlp.GRPCServer = &mockLogsReceiver{}
+
 type mockLogsReceiver struct {
 	plogotlp.UnimplementedGRPCServer
 	mockReceiver
@@ -177,6 +181,8 @@ func otlpLogsReceiverOnGRPCServer(ln net.Listener) *mockLogsReceiver {
 	return rcv
 }
 
+var _ pmetricotlp.GRPCServer = &mockMetricsReceiver{}
+
 type mockMetricsReceiver struct {
 	pmetricotlp.UnimplementedGRPCServer
 	mockReceiver