Wire Prometheus metric export to state V1 APIs

Extends metric producer classes with the requested exposition format.
As a consequence, the State API server has been changed to allow
emitting other content types than just `application/json`.

Add custom Prometheus rendering for Slobrok, as it does its own
domain-specific metric tracking. However, since it has non-destructive
sampling properties, we can actually use proper `counter` types.

3 files changed, 64 insertions, 24 deletions

diff --git a/metrics/src/tests/metricmanagertest.cpp b/metrics/src/tests/metricmanagertest.cpp
index e5e39bd9dcb..9629c63f333 100644
--- a/metrics/src/tests/metricmanagertest.cpp
+++ b/metrics/src/tests/metricmanagertest.cpp
@@ -572,7 +572,7 @@ TEST_F(MetricManagerTest, test_json_output)
         // No snapshots have been taken yet, so the non-total getMetrics call should return
         // the empty string (i.e. no metrics produced).
         metrics::StateApiAdapter adapter(mm);
-        auto json_str = adapter.getMetrics("snapper");
+        auto json_str = adapter.getMetrics("snapper", vespalib::MetricsProducer::ExpositionFormat::JSON);
         EXPECT_EQ(json_str, "");
     }
 
@@ -635,9 +635,9 @@ TEST_F(MetricManagerTest, test_json_output)
     EXPECT_EQ(10.0, slime.get()["values"][10]["values"]["last"].asDouble()) << jsonData;
 
     metrics::StateApiAdapter adapter(mm);
-    vespalib::string normal = adapter.getMetrics("snapper");
+    vespalib::string normal = adapter.getMetrics("snapper", vespalib::MetricsProducer::ExpositionFormat::JSON);
     EXPECT_EQ(vespalib::string(jsonData), normal);
-    vespalib::string total = adapter.getTotalMetrics("snapper");
+    vespalib::string total = adapter.getTotalMetrics("snapper", vespalib::MetricsProducer::ExpositionFormat::JSON);
     EXPECT_GT(total.size(), 0);
     EXPECT_NE(total, normal);
 }
@@ -1058,6 +1058,18 @@ TEST_F(MetricManagerTest, prometheus_output_can_emit_inf_values_verbatim) {
     EXPECT_THAT(actual, HasSubstr("outer_temp_val_sum{foo=\"baz\",fancy=\"stuff\"} -Inf 1300000\n"));
 }
 
+TEST_F(MetricManagerTest, state_adapter_can_output_prometheus_format) {
+    SameNamesTestMetricSet mset;
+    mset.set1.val.addValue(2);
+    mset.set2.val.addValue(3);
+    MetricSnapshotTestFixture fixture(*this, mset);
+    fixture.takeSnapshotsOnce();
+    metrics::StateApiAdapter adapter(fixture.manager);
+    auto metrics = adapter.getMetrics("snapper", vespalib::MetricsProducer::ExpositionFormat::Prometheus);
+    EXPECT_THAT(metrics, HasSubstr("outer_temp_val_sum{foo=\"bar\",fancy=\"stuff\"} 2 1300000\n"));
+    EXPECT_THAT(metrics, HasSubstr("outer_temp_val_sum{foo=\"baz\",fancy=\"stuff\"} 3 1300000\n"));
+}
+
 struct SneakyNamesMetricSet : public MetricSet {
     DoubleValueMetric val1;
     DoubleValueMetric val2;
diff --git a/metrics/src/vespa/metrics/state_api_adapter.cpp b/metrics/src/vespa/metrics/state_api_adapter.cpp
index 56a04542345..2c92448fe95 100644
--- a/metrics/src/vespa/metrics/state_api_adapter.cpp
+++ b/metrics/src/vespa/metrics/state_api_adapter.cpp
@@ -1,30 +1,45 @@
 // Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
 #include "jsonwriter.h"
-#include "state_api_adapter.h"
 #include "metricmanager.h"
+#include "prometheus_writer.h"
+#include "state_api_adapter.h"
 #include <vespa/vespalib/stllike/asciistream.h>
 
 namespace metrics {
 
 vespalib::string
-StateApiAdapter::getMetrics(const vespalib::string &consumer)
+StateApiAdapter::getMetrics(const vespalib::string &consumer, ExpositionFormat format)
 {
     MetricLockGuard guard(_manager.getMetricLock());
     auto periods = _manager.getSnapshotPeriods(guard);
     if (periods.empty() || !_manager.any_snapshots_taken(guard)) {
         return ""; // no configuration or snapshots yet
     }
-    const MetricSnapshot &snapshot(_manager.getMetricSnapshot(guard, periods[0]));
-    vespalib::asciistream json;
-    vespalib::JsonStream stream(json);
-    metrics::JsonWriter metricJsonWriter(stream);
-    _manager.visit(guard, snapshot, metricJsonWriter, consumer);
-    stream.finalize();
-    return json.str();
+    const MetricSnapshot& snapshot(_manager.getMetricSnapshot(guard, periods[0]));
+    vespalib::asciistream out;
+    // Using `switch` instead of `if` so that we fail with a compiler warning -> error if
+    // we add another enum value and forget to add a case for it here.
+    switch (format) {
+    case ExpositionFormat::JSON:
+        {
+            vespalib::JsonStream stream(out);
+            JsonWriter metricJsonWriter(stream);
+            _manager.visit(guard, snapshot, metricJsonWriter, consumer);
+            stream.finalize();
+            break;
+        }
+    case ExpositionFormat::Prometheus:
+        {
+            PrometheusWriter writer(out);
+            _manager.visit(guard, snapshot, writer, consumer);
+            break;
+        }
+    }
+    return out.str();
 }
 
 vespalib::string
-StateApiAdapter::getTotalMetrics(const vespalib::string &consumer)
+StateApiAdapter::getTotalMetrics(const vespalib::string &consumer, ExpositionFormat format)
 {
     _manager.updateMetrics();
     MetricLockGuard guard(_manager.getMetricLock());
@@ -34,13 +49,26 @@ StateApiAdapter::getTotalMetrics(const vespalib::string &consumer)
                                                       _manager.getTotalMetricSnapshot(guard).getMetrics(), true);
     _manager.getActiveMetrics(guard).addToSnapshot(*generated, false, currentTime);
     generated->setFromTime(_manager.getTotalMetricSnapshot(guard).getFromTime());
-    const MetricSnapshot &snapshot = *generated;
-    vespalib::asciistream json;
-    vespalib::JsonStream stream(json);
-    metrics::JsonWriter metricJsonWriter(stream);
-    _manager.visit(guard, snapshot, metricJsonWriter, consumer);
-    stream.finalize();
-    return json.str();
+    const MetricSnapshot& snapshot = *generated;
+    vespalib::asciistream out;
+    switch (format) {
+    case ExpositionFormat::JSON:
+        {
+            vespalib::JsonStream stream(out);
+            metrics::JsonWriter metricJsonWriter(stream);
+            _manager.visit(guard, snapshot, metricJsonWriter, consumer);
+            stream.finalize();
+            break;
+        }
+    case ExpositionFormat::Prometheus:
+        {
+            PrometheusWriter writer(out);
+            _manager.visit(guard, snapshot, writer, consumer);
+            break;
+        }
+    }
+
+    return out.str();
 }
 
 } // namespace metrics
diff --git a/metrics/src/vespa/metrics/state_api_adapter.h b/metrics/src/vespa/metrics/state_api_adapter.h
index fd610226fda..39a80099355 100644
--- a/metrics/src/vespa/metrics/state_api_adapter.h
+++ b/metrics/src/vespa/metrics/state_api_adapter.h
@@ -11,7 +11,7 @@ class MetricManager;
 /**
  * This is an adapter class that implements the metrics producer
  * interface defined by the state api implementation in vespalib by
- * extracting metrics in json format from a metric manager.
+ * extracting metrics in JSON or Prometheus format from a metric manager.
  **/
 class StateApiAdapter : public vespalib::MetricsProducer
 {
@@ -19,10 +19,10 @@ private:
     MetricManager &_manager;
 
 public:
-    StateApiAdapter(MetricManager &manager) : _manager(manager) {}
+    explicit StateApiAdapter(MetricManager &manager) : _manager(manager) {}
 
-    vespalib::string getMetrics(const vespalib::string &consumer) override;
-    vespalib::string getTotalMetrics(const vespalib::string &consumer) override;
+    vespalib::string getMetrics(const vespalib::string &consumer, ExpositionFormat format) override;
+    vespalib::string getTotalMetrics(const vespalib::string &consumer, ExpositionFormat format) override;
 };
 
 } // namespace metrics