feat: Add load balancer prometheus metrics (#2096)

Fix: https://github.com/XRPLF/clio/issues/2070

src/etl/LoadBalancer.hpp

@@ -32,6 +32,8 @@
 #include "util/ResponseExpirationCache.hpp"
 #include "util/log/Logger.hpp"
 #include "util/newconfig/ConfigDefinition.hpp"
+#include "util/prometheus/Counter.hpp"
+#include "util/prometheus/Histogram.hpp"
 
 #include <boost/asio.hpp>
 #include <boost/asio/io_context.hpp>
@@ -92,7 +94,12 @@ private:
     std::uint32_t downloadRanges_ =
         kDEFAULT_DOWNLOAD_RANGES; /*< The number of markers to use when downloading initial ledger */
 
-    // Using mutext instead of atomic_bool because choosing a new source to
+    std::reference_wrapper<util::prometheus::HistogramInt> forwardedDurationHistogram_;
+    std::reference_wrapper<util::prometheus::CounterInt> forwardedRetryCounter_;
+    std::reference_wrapper<util::prometheus::CounterInt> cacheTriedCounter_;
+    std::reference_wrapper<util::prometheus::CounterInt> cacheMissCounter_;
+
+    // Using mutex instead of atomic_bool because choosing a new source to
     // forward messages should be done with a mutual exclusion otherwise there will be a race condition
     util::Mutex<bool> hasForwardingSource_{false};
 
@@ -133,7 +140,7 @@ public:
      * @param ioc The io_context to run on
      * @param backend BackendInterface implementation
      * @param subscriptions Subscription manager
-     * @param validatedLedgers The network validated ledgers datastructure
+     * @param validatedLedgers The network validated ledgers data structure
      * @param sourceFactory A factory function to create a source
      * @return A shared pointer to a new instance of LoadBalancer
      */
@@ -147,8 +154,6 @@ public:
         SourceFactory sourceFactory = makeSource
     );
 
-    ~LoadBalancer() override;
-
     /**
      * @brief Load the initial ledger, writing data to the queue.
      * @note This function will retry indefinitely until the ledger is downloaded.