Publish metrics also for metricbeat

stats_types/src/stats_types.rs

@@ -1,3 +1,5 @@
+pub use serde_json;
+
 use std::sync::atomic::AtomicU64;
 use std::sync::atomic::Ordering::AcqRel;
 use std::sync::atomic::Ordering::Acquire;
@@ -192,6 +194,10 @@ impl HistoLog2 {
         ret.push_str("\n");
         ret
     }
+
+    pub fn to_json(&self, _name: &str) -> serde_json::Value {
+        serde_json::Value::Null
+    }
 }
 
 #[test]