Set FIELD_AUTOFILL_NUM_DATASETS to -1 when service return no response.

Test: adb shell logcat -b events | grep sysui | grep '757,907'
     # while running LoginActivityTest#testAutoFillNoDatasets

Bug: 70241978

Change-Id: I611bd4fa8407b16491f1ae7b859564233fa6ba81
diff --git a/proto/src/metrics_constants.proto b/proto/src/metrics_constants.proto
index 0cc60e3..b3ec8d0 100644
--- a/proto/src/metrics_constants.proto
+++ b/proto/src/metrics_constants.proto
@@ -3964,7 +3964,8 @@
     // Type TYPE_FAILURE: The request failed
     // Package: Package of app that is autofilled
     // Tag FIELD_AUTOFILL_SERVICE: Package of service that processed the request
-    // Tag FIELD_AUTOFILL_NUM_DATASETS: The number of datasets returned (only in success case)
+    // Tag FIELD_AUTOFILL_NUM_DATASETS: The number of datasets returned in the response, or -1 if
+    // the service returned a null response.
     // NOTE: starting on OS P, it also added:
     // Type TYPE_CLOSE: Service returned a null response.
     // Tag FIELD_AUTOFILL_NUM_FIELD_CLASSIFICATION_IDS: if service requested field classification,