MB-43772: [BP] Only collect exposer_ metrics for the low cardinality

Backport to 7.2.5.

This is because ns_server will concat the outputs from both the low
cardinality and high cardinality endpoints. Collecting exposer_ metrics
for both results in duplicate metrics after the concat, which is not
allowed by the Prometheus exposition format.

Change-Id: Ifcf0e51e8dbf08bf7a8f4189b8a5e6c0c9ec7d2e
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/203920
Reviewed-by: Trond Norbye <[email protected]>
Well-Formed: Restriction Checker
Tested-by: Vesko Karaganev <[email protected]>

include/statistics/prometheus.h

@@ -44,6 +44,15 @@ enum class IncludeTimestamps {
     Yes,
 };
 
+/**
+ * Flag whether to include meta metrics generated by prometheus-cpp for every
+ * endpoint.
+ *
+ * Those metrics start with "exposer_" and contain information about metrics
+ * scape latencies, among others.
+ */
+enum class IncludeMetaMetrics { No, Yes };
+
 using AuthCallback =
         std::function<bool(const std::string&, const std::string&)>;
 

statistics/prometheus.cc

@@ -176,11 +176,19 @@ MetricServer::MetricServer(in_port_t port,
 
         // used to store the newly created KVCollectable into the array
         auto arrayItr = endpoints.begin();
-        for (auto [cardinality, timestamps] : {
-                     std::make_pair(Cardinality::Low, IncludeTimestamps::No),
-                     std::make_pair(Cardinality::Low, IncludeTimestamps::Yes),
-                     std::make_pair(Cardinality::High, IncludeTimestamps::No),
-                     std::make_pair(Cardinality::High, IncludeTimestamps::Yes),
+        for (auto [cardinality, timestamps, metaMetrics] : {
+                     std::make_tuple(Cardinality::Low,
+                                     IncludeTimestamps::No,
+                                     IncludeMetaMetrics::Yes),
+                     std::make_tuple(Cardinality::Low,
+                                     IncludeTimestamps::Yes,
+                                     IncludeMetaMetrics::Yes),
+                     std::make_tuple(Cardinality::High,
+                                     IncludeTimestamps::No,
+                                     IncludeMetaMetrics::No),
+                     std::make_tuple(Cardinality::High,
+                                     IncludeTimestamps::Yes,
+                                     IncludeMetaMetrics::No),
              }) {
             auto ptr = std::make_shared<KVCollectable>(
                     cardinality, timestamps, getStatsCB);
@@ -195,6 +203,12 @@ MetricServer::MetricServer(in_port_t port,
 
             exposer->RegisterAuth(authCB, authRealm, path);
             exposer->RegisterCollectable(ptr, path);
+
+            if (metaMetrics == IncludeMetaMetrics::No) {
+                exposer->RemoveCollectable(exposer->GetMetaCollectable(path),
+                                           path);
+            }
+
             *arrayItr = std::move(ptr);
             ++arrayItr;
         }