implement the fix

lib/instrumentation/pino/pino.js

@@ -131,7 +131,15 @@ function reformatLogLine({ logLine, msg, agent, chindings = '', level, logger })
     delete metadata.hostname
   }
 
-  const agentMeta = Object.assign({}, { timestamp: Date.now(), message: msg }, metadata)
+  const agentMeta = Object.assign({}, { timestamp: Date.now() }, metadata)
+  // eslint-disable-next-line eqeqeq
+  if (msg != undefined) {
+    // The spec lists `message` as "MUST" under the required column, but then
+    // details that it "MUST be omitted" if the value is "empty". Additionally,
+    // if someone has logged only a merging object, and that object contains a
+    // message key, we do not want to overwrite their value. See issue 2595.
+    agentMeta.message = msg
+  }
 
   /**
    * A function that gets executed in `_toPayloadSync` of log aggregator.