improve the logging for this error: warn so it isn't missed, details …

…at trace level

CHANGELOG.asciidoc

@@ -46,6 +46,11 @@ See the <<upgrade-to-v4>> guide.
 [float]
 ===== Bug fixes
 
+* Guard against a possible encoding error of tracing data in the APM client,
+  before it is sent. It is *possible* this could wedge the APM client,
+  resulting in the APM agent no longer sending tracing data.
+  ({pull}4359[#4359])
+
 [float]
 ===== Chores
 

lib/apm-client/http-apm-client/index.js

@@ -740,7 +740,16 @@ Client.prototype._encode = function (obj, enc) {
   try {
     out[outAttr] = truncFunc(thing, this._conf);
   } catch (err) {
-    this._log.trace({ err, thing }, `error encoding "${outAttr}" object`);
+    this._log.warn(
+      {
+        err,
+        // Only log full problematic object at TRACE level to limit noise.
+        thing: this._log.isLevelEnabled('trace') ? thing : '[REDACTED]',
+        thing_id: thing?.id,
+        thing_name: thing?.name,
+      },
+      `could not encode "${outAttr}" object`,
+    );
     return null;
   }