SERVER-63642 Add serverStatus histogram metrics to measure multi-planning performance

1 files changed, 44 insertions, 0 deletions

diff --git a/src/mongo/util/histogram.h b/src/mongo/util/histogram.h
index 0a07c82d9bc..78ba29664ba 100644
--- a/src/mongo/util/histogram.h
+++ b/src/mongo/util/histogram.h
@@ -184,4 +184,48 @@ void appendHistogram(BSONObjBuilder& bob, const Histogram<T>& hist, const String
     histBob.append("totalCount", totalCount);
 }
 
+namespace histogram_detail {
+/**
+ * Append the histogram as an array of {bound: <lower>, count: <count>} objects.
+ */
+template <typename T>
+void appendHistogramAsArray(const Histogram<T>& hist,
+                            const StringData histKey,
+                            bool includeFirstBucket,
+                            BSONObjBuilder& bob) {
+    BSONArrayBuilder histBob(bob.subarrayStart(histKey));
+
+    for (auto&& [count, lower, upper] : hist) {
+        // First bucket is indicated by 'lower' = nullptr.
+        if (lower) {
+            histBob.append(
+                BSON("lowerBound" << static_cast<long long>(*lower) << "count" << count));
+        } else if (includeFirstBucket) {
+            histBob.append(BSON("lowerBound"
+                                << "-INF"
+                                << "count" << count));
+        }
+    }
+}
+}  // namespace histogram_detail
+
+/**
+ * A metric reported in serverStatus with type histogram. This is a partial specialization of
+ * ServerStatusMetricField<T>. We cannot include it there due to cyclic header dependency via
+ * "commands.h".
+ */
+template <typename T>
+class ServerStatusMetricField<Histogram<T>> : public ServerStatusMetric {
+public:
+    ServerStatusMetricField(const std::string& name, const Histogram<T>& t)
+        : ServerStatusMetric(name), _hist(t) {}
+
+    virtual void appendAtLeaf(BSONObjBuilder& b) const {
+        histogram_detail::appendHistogramAsArray(_hist, _leafName, false, b);
+    }
+
+private:
+    const Histogram<T>& _hist;
+};
+
 }  // namespace mongo