histogram_metrics_helper.hh: Add to_metrics_summary function

The to_metrics_summary is a helper function that create a metrics type
summary from a timed_rate_moving_average_with_summary object.

Signed-off-by: Amnon Heiman <amnon@scylladb.com>

configure.py

@@ -704,6 +704,7 @@ scylla_core = (['replica/database.cc',
                 'utils/generation-number.cc',
                 'utils/rjson.cc',
                 'utils/human_readable.cc',
+                'utils/histogram_metrics_helper.cc',
                 'mutation_partition.cc',
                 'mutation_partition_view.cc',
                 'mutation_partition_serializer.cc',

utils/histogram_metrics_helper.cc

@@ -0,0 +1,20 @@
+/*
+ * Copyright (C) 2022-present ScyllaDB
+ */
+
+/*
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+#include "histogram_metrics_helper.hh"
+
+seastar::metrics::histogram to_metrics_summary(const utils::summary_calculator& summary) noexcept {
+    seastar::metrics::histogram res;
+    res.buckets.resize(summary.quantiles().size());
+    res.sample_count = summary.histogram().count();
+    for (size_t i = 0; i < summary.quantiles().size(); i++) {
+        res.buckets[i].count = summary.summary()[i];
+        res.buckets[i].upper_bound = summary.quantiles()[i];
+    }
+    return res;
+}

utils/histogram_metrics_helper.hh

@@ -14,7 +14,7 @@
 #include <seastar/core/metrics_types.hh>
 #include "seastarx.hh"
 #include "estimated_histogram.hh"
-
+#include "histogram.hh"
 
 template<uint64_t Min, uint64_t Max, size_t Precision>
 seastar::metrics::histogram to_metrics_histogram(const utils::approx_exponential_histogram<Min, Max, Precision>& hist) {
@@ -35,3 +35,12 @@ seastar::metrics::histogram to_metrics_histogram(const utils::approx_exponential
     res.sample_sum += hist.get(hist.NUM_BUCKETS - 1) * hist.get_bucket_lower_limit(hist.NUM_BUCKETS - 1);
     return res;
 }
+
+/*!
+ * \brief get a metrics summary from timed_rate_moving_average_with_summary
+ *
+ * timed_rate_moving_average_with_summary contains a summary. This function
+ * copy it to a metric summary.
+ * A metric summary is a histogram where each bucket holds some quantile.
+ */
+seastar::metrics::histogram to_metrics_summary(const utils::summary_calculator& summary) noexcept;