Add pipeline metrics to Node Stats API (#16839)

This commit introduces three new metrics per pipeline in the Node Stats API:
- workers
- batch_size
- batch_delay

```
{
  ...
  pipelines: {
    main: {
      events: {...}, 
      flow: {...}, 
      plugins: {...}, 
      reloads: {...}, 
      queue: {...}, 
      pipeline: {
        workers: 12,
        batch_size: 125,
        batch_delay: 5,
      }, 
    }
  }
  ...
}
```

docs/static/monitoring/monitoring-apis.asciidoc

@@ -726,6 +726,11 @@ Example response:
       },
       "queue" : {
         "type" : "memory"
+      },
+      "pipeline": {
+        "workers": 4,
+        "batch_size": 125,
+        "batch_delay": 50
       }
     },
     "test2" : {
@@ -807,6 +812,11 @@ Example response:
       },
       "queue" : {
         "type" : "memory"
+      },
+      "pipeline": {
+        "workers": 4,
+        "batch_size": 125,
+        "batch_delay": 50
       }
     }
   }
@@ -969,6 +979,11 @@ Example response:
         "events_count": 0,
         "queue_size_in_bytes": 3885,
         "max_queue_size_in_bytes": 1073741824
+      },
+      "pipeline": {
+        "workers": 4,
+        "batch_size": 125,
+        "batch_delay": 50
       }
     }
   }

logstash-core/lib/logstash/api/commands/stats.rb

@@ -183,7 +183,12 @@ def report(stats, extended_stats = nil, opts = {})
                 :outputs => plugin_stats(stats, :outputs)
               },
               :reloads => stats[:reloads],
-              :queue => stats[:queue]
+              :queue => stats[:queue],
+              :pipeline => {
+                :workers => stats[:config][:workers],
+                :batch_size => stats[:config][:batch_size],
+                :batch_delay => stats[:config][:batch_delay],
+              }
             }
             ret[:dead_letter_queue] = stats[:dlq] if stats.include?(:dlq)
 

logstash-core/spec/logstash/api/commands/stats_spec.rb

@@ -162,6 +162,7 @@
           :plugins,
           :reloads,
           :queue,
+          :pipeline
         )
       end
       it "returns event information" do