MB-66329: Expose FusionNamespace for a bucket as a stat

Change-Id: I54a0c2604796c8d25151b37bd1d0d48aa5c25bb0
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/226689
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Vesko Karaganev <vesko.karaganev@couchbase.com>

Author: mohammadzaeem

engines/ep/src/ep_engine.cc

@@ -3295,6 +3295,7 @@ void EventuallyPersistentEngine::doEngineStatsFusion(
         const StatCollector& collector) {
     using namespace cb::stats;
 
+    // getStats from Magma
     constexpr std::array<std::string_view, 17> statNames = {
             {"fusion_NumSyncs",
              "fusion_NumBytesSynced",
@@ -3356,6 +3357,11 @@ void EventuallyPersistentEngine::doEngineStatsFusion(
     addStat(Key::ep_fusion_file_map_mem_used, "fusion_FileMapMemUsed");
     addStat(Key::ep_fusion_sync_failures, "fusion_NumSyncFailures");
     addStat(Key::ep_fusion_migration_failures, "fusion_NumMigrationFailures");
+
+    // Additional Fusion Stats
+    collector.addStat(Key::ep_fusion_namespace,
+                      configuration.getMagmaFusionNamespacePrefix() + "/" +
+                              name + "/" + configuration.getUuid());
 }
 
 cb::engine_errc EventuallyPersistentEngine::doEngineStats(

statistics/stat_definitions.json

@@ -1806,6 +1806,13 @@
         "added": "7.2.0"
     },
 /* Fusion Stats */
+    {
+        "key": "ep_fusion_namespace",
+        "unit": "none",
+        "prometheus": false,
+        "description": "The namespace corresponds to an instance of a Couchbase bucket. It is in the form '<service_prefix>/<bucket_name>/<bucket_uuid>'",
+        "added": "8.0.0"
+    },
     {
         "key": "ep_fusion_syncs",
         "unit": "count",

tests/testapp/testapp_fusion.cc

@@ -447,7 +447,8 @@ TEST_P(FusionTest, Stat_UploaderState_KVStoreInvalid) {
 }
 
 TEST_P(FusionTest, GetPrometheusFusionStats) {
-    std::array<std::string_view, 17> statKeysExpected = {
+    std::array<std::string_view, 18> statKeysExpected = {
+            "ep_fusion_namespace",
             "ep_fusion_syncs",
             "ep_fusion_bytes_synced",
             "ep_fusion_logs_migrated",