Add oldest version and newest version metrics

ConflictSet.cpp

@@ -3156,6 +3156,7 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
     }
 
     newestVersionFullPrecision = writeVersion;
+    newest_version.set(newestVersionFullPrecision);
     setOldestVersion(
         std::max(oldestVersionFullPrecision,
                  newestVersionFullPrecision - kNominalVersionWindow));
@@ -3265,6 +3266,7 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
     nodes_released_total.add(std::exchange(nodes_released_accum, 0));
     entries_inserted_total.add(std::exchange(entries_inserted_accum, 0));
     entries_erased_total.add(std::exchange(entries_erased_accum, 0));
+    oldest_version.set(oldestVersionFullPrecision);
   }
 
   int64_t getBytes() const { return totalBytes; }
@@ -3273,6 +3275,8 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
     this->oldestVersion = InternalVersionT(oldestVersion);
     oldestVersionFullPrecision = oldestExtantVersion = oldestVersionAtGcBegin =
         newestVersionFullPrecision = oldestVersion;
+    oldest_version.set(oldestVersionFullPrecision);
+    newest_version.set(newestVersionFullPrecision);
 
     allocators.~NodeAllocators();
     new (&allocators) NodeAllocators();
@@ -3387,6 +3391,9 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
       gc_iterations_total,
       "The total number of iterations of the main loop for garbage collection");
   COUNTER(write_bytes_total, "Total number of key bytes in calls to addWrites");
+  GAUGE(oldest_version,
+        "The lowest version that doesn't result in \"TooOld\" for checks");
+  GAUGE(newest_version, "The version of the most recent call to addWrites");
 #if MEASURE_CHECK_CPU_TIME
   COUNTER(check_cpu_seconds_total,
           "Total cpu seconds spent in a call to check");