jinx: add metric for empty candidates

As outlined in the linked bug, we have seen some odd behaviour with
candidates added to text suggestion requests when debugging. This CL
adds a new metric in light of this to ensure this is no a widespread
issue in our experiment user population.

BUG=b:284768533

Change-Id: I4f1fdb534f4eef1ea08b27f2323eec91175f5243
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4572745
Commit-Queue: Curtis McMullan <curtismcmullan@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1150185}

chrome/browser/ash/input_method/suggestions_service_client.cc

@@ -8,6 +8,7 @@
 #include "base/functional/bind.h"
 #include "base/metrics/field_trial_params.h"
 #include "base/metrics/histogram_functions.h"
+#include "base/metrics/histogram_macros.h"
 #include "chrome/browser/ash/input_method/suggestion_enums.h"
 #include "chromeos/services/machine_learning/public/cpp/service_connection.h"
 #include "third_party/abseil-cpp/absl/types/optional.h"
@@ -105,6 +106,11 @@ void RecordRequestCandidates(
       ToSuggestionType(suggestion_mode));
 }
 
+void RecordEmptyCandidate(const ime::AssistiveSuggestionMode& suggestion_mode) {
+  UMA_HISTOGRAM_ENUMERATION("InputMethod.Assistive.MultiWord.EmptyCandidate",
+                            ToSuggestionType(suggestion_mode));
+}
+
 void RecordCandidatesGenerated(AssistiveSuggestionMode suggestion_mode) {
   base::UmaHistogramEnumeration(
       "InputMethod.Assistive.MultiWord.CandidatesGenerated",
@@ -155,6 +161,9 @@ void SuggestionsServiceClient::RequestSuggestions(
     auto next_word_candidate = NextWordCompletionCandidate::New();
     next_word_candidate->text = candidate.text;
     next_word_candidate->normalized_score = candidate.score;
+    if (next_word_candidate->text.empty()) {
+      RecordEmptyCandidate(suggestion_mode);
+    }
     query->next_word_candidates.push_back(std::move(next_word_candidate));
   }
 

chrome/browser/ash/input_method/suggestions_service_client_unittest.cc

@@ -306,6 +306,36 @@ TEST_F(SuggestionsServiceClientTest,
       /*expected_bucket_count=*/1);
 }
 
+TEST_F(SuggestionsServiceClientTest,
+       RecordsEmptyCandidateTextWhenCandidateTextMissing) {
+  SetTextSuggesterResult(SingleCandidate("hi there completion", 0.5f));
+
+  base::HistogramTester histogram_tester;
+  histogram_tester.ExpectTotalCount(
+      "InputMethod.Assistive.MultiWord.EmptyCandidate", 0);
+
+  client()->RequestSuggestions(
+      /*preceding_text=*/"hel",
+      /*suggestion_mode=*/AssistiveSuggestionMode::kCompletion,
+      /*completion_candidates=*/
+      std::vector<DecoderCompletionCandidate>{
+          DecoderCompletionCandidate{"hello", 0.1f},
+          DecoderCompletionCandidate{"", 0.01f},
+          DecoderCompletionCandidate{"", 0.001f},
+      },
+      /*callback=*/
+      base::BindLambdaForTesting(
+          [&](const std::vector<AssistiveSuggestion>& results) {}));
+  WaitForResults();
+
+  histogram_tester.ExpectTotalCount(
+      "InputMethod.Assistive.MultiWord.EmptyCandidate", 2);
+  histogram_tester.ExpectUniqueSample(
+      "InputMethod.Assistive.MultiWord.EmptyCandidate",
+      /*sample=*/MultiWordSuggestionType::kCompletion,
+      /*expected_bucket_count=*/2);
+}
+
 }  // namespace
 }  // namespace input_method
 }  // namespace ash

tools/metrics/histograms/metadata/input/histograms.xml

@@ -657,6 +657,19 @@ chromium-metrics-reviews@google.com.
   </summary>
 </histogram>
 
+<histogram name="InputMethod.Assistive.MultiWord.EmptyCandidate"
+    enum="IMEAssistiveMultiWordSuggestionType" expires_after="2023-10-01">
+  <owner>curtismcmullan@google.com</owner>
+  <owner>essential-inputs-team@google.com</owner>
+  <summary>
+    This histogram records when an empty completion candidate is added to a
+    suggestion generation request to the machine learning service. An empty
+    completion candidate in this context means a candidate with an empty string.
+    This is an invalid state and should never happen. The metric is recorded
+    once per empty candidate found.
+  </summary>
+</histogram>
+
 <histogram name="InputMethod.Assistive.MultiWord.ImplicitAcceptance"
     enum="IMEAssistiveMultiWordSuggestionType" expires_after="2023-10-01">
   <owner>curtismcmullan@google.com</owner>