cpu_scheduling: add scheduling_groups probe

This patch introduces a probe that queries each scheduling group
for its usage stats and publishes metrics based on that.

The following new metric is introduced:
Name: redpanda_scheduler_runtime_seconds
Description: Accumulated runtime of task queue associated with this
scheduling group
Labels:
  - redpanda_scheduling_group
  - shard

src/v/resource_mgmt/scheduling_groups_probe.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright 2020 Redpanda Data, Inc.
+ *
+ * Use of this software is governed by the Business Source License
+ * included in the file licenses/BSL.md
+ *
+ * As of the Change Date specified in that file, in accordance with
+ * the Business Source License, use of this software will be governed
+ * by the Apache License, Version 2.0
+ */
+
+#pragma once
+
+#include "cluster/partition_leaders_table.h"
+#include "config/configuration.h"
+#include "prometheus/prometheus_sanitize.h"
+#include "resource_mgmt/cpu_scheduling.h"
+#include "ssx/metrics.h"
+
+#include <seastar/core/metrics.hh>
+
+class scheduling_groups_probe {
+public:
+    void wire_up(const scheduling_groups& scheduling_groups) {
+        if (config::shard_local_cfg().disable_public_metrics()) {
+            return;
+        }
+
+        auto groups = scheduling_groups.all_scheduling_groups();
+        for (auto&& group_ref : groups) {
+            _public_metrics.add_group(
+              prometheus_sanitize::metrics_name("scheduler"),
+              {seastar::metrics::make_gauge(
+                "runtime_seconds",
+                [group_ref] {
+                    return group_ref.get().get_stats().runtime.count();
+                },
+                seastar::metrics::description(
+                  "Accumulated runtime of task queue associated with this "
+                  "scheduling group"),
+                {ssx::metrics::make_namespaced_label("scheduling_group")(
+                  group_ref.get().name())})});
+        }
+    }
+
+    void clear() { _public_metrics.clear(); }
+
+private:
+    seastar::metrics::metric_groups _public_metrics{
+      ssx::metrics::public_metrics_handle};
+};