fix(config): keep the lookup value as passed

The previous version was transforming the lookup
parameter to a string.

Closes #4894

Author: blemoine

packages/dd-trace/src/config.js

@@ -992,7 +992,7 @@ class Config {
     this._setBoolean(opts, 'llmobs.agentlessEnabled', options.llmobs?.agentlessEnabled)
     this._setString(opts, 'llmobs.mlApp', options.llmobs?.mlApp)
     this._setBoolean(opts, 'logInjection', options.logInjection)
-    this._setString(opts, 'lookup', options.lookup)
+    this._setValue(opts, 'lookup', options.lookup)
     this._setBoolean(opts, 'middlewareTracingEnabled', options.middlewareTracingEnabled)
     this._setBoolean(opts, 'openAiLogsEnabled', options.openAiLogsEnabled)
     this._setValue(opts, 'peerServiceMapping', options.peerServiceMapping)

packages/dd-trace/test/config.spec.js

@@ -1614,6 +1614,13 @@ describe('Config', () => {
     expect(config.tags).to.include({ foo: 'bar', baz: 'qux' })
   })
 
+  it('should not transform the lookup parameter', () => {
+    const lookup = () => 'test'
+    const config = new Config({ lookup: lookup })
+
+    expect(config.lookup).to.equal(lookup)
+  })
+
   it('should not set DD_INSTRUMENTATION_TELEMETRY_ENABLED if AWS_LAMBDA_FUNCTION_NAME is present', () => {
     process.env.AWS_LAMBDA_FUNCTION_NAME = 'my-great-lambda-function'