fix: adapt logger.child call to new pino 6 format (#2333)

This avoids a possible PINODEP004 warning if a given custom logger
is Pino >=6.0.0 <6.12.0.

Fixes #2332

CHANGELOG.asciidoc

@@ -68,6 +68,8 @@ This fixes an issue with context binding of EventEmitters, where
 `removeListener` would fail to actually remove if the same handler function was
 added to multiple events.
 
+* Fix pino's deprecation warning when using a custom logger with pino@6 ({issues}2332[#2332])
+
 
 [[release-notes-3.23.0]]
 ==== 3.23.0 2021/10/25

lib/logging.js

@@ -30,6 +30,7 @@
 
 var ecsFormat = require('@elastic/ecs-pino-format')
 var pino = require('pino')
+var semver = require('semver')
 
 const DEFAULT_LOG_LEVEL = 'info'
 
@@ -116,6 +117,11 @@ function createLogger (levelName, customLogger) {
     // Is this a pino logger? If so, it supports the API the agent requires and
     // can be used directly. We must add our custom serializers.
     if (Symbol.for('pino.serializers') in customLogger) {
+      // Pino added `options` second arg to `logger.child` in 6.12.0.
+      if (semver.gte(customLogger.version, '6.12.0')) {
+        return customLogger.child({}, { serializers })
+      }
+
       return customLogger.child({
         serializers: serializers
       })