feat(oohelperd): add prometheus metrics (#897)

Closes https://github.com/ooni/probe/issues/2183

While there, avoid exposing nil values for optional fields of the
THResponse struct (i.e., "ip_info" and "tls_handshake").

While there, fix `measurexlite`'s `OperationLogger` test
and make it deterministic rather than racy.

internal/measurexlite/logger_test.go

@@ -72,7 +72,7 @@ func TestNewOperationLogger(t *testing.T) {
 		}
 		const maxwait = 100 * time.Microsecond
 		ol := newOperationLogger(maxwait, logger, "antani%d", 0)
-		time.Sleep(4 * ol.maxwait)
+		ol.wg.Wait() // wait for the message to be emitted
 		ol.Stop(nil)
 		if len(lines) != 2 {
 			t.Fatal("unexpected number of lines")
@@ -100,7 +100,7 @@ func TestNewOperationLogger(t *testing.T) {
 		}
 		const maxwait = 100 * time.Microsecond
 		ol := newOperationLogger(maxwait, logger, "antani%d", 0)
-		time.Sleep(4 * ol.maxwait)
+		ol.wg.Wait() // wait for the message to be emitted
 		ol.Stop(io.EOF)
 		if len(lines) != 2 {
 			t.Fatal("unexpected number of lines")