Revert "deepmerge in merging apm-configuration (#179048)" (#180005)

## Summary

Reverting #179048 because it creates a false expectation that
environment vars and config will be merged. This is true for the RUM
agent. However, the NodeJS agent reads the environment vars and discards
any provided config, leading to inconsistent behavior between both
agents.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### For maintainers

- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

packages/kbn-apm-config-loader/src/config.test.ts

@@ -7,9 +7,9 @@
  */
 import type { AgentConfigOptions, Labels } from 'elastic-apm-node';
 import {
-  gitRevExecMock,
-  mockedRootDir,
   packageMock,
+  mockedRootDir,
+  gitRevExecMock,
   readUuidFileMock,
   resetAllMocks,
 } from './config.test.mocks';
@@ -153,7 +153,6 @@ describe('ApmConfiguration', () => {
       delete process.env.ELASTIC_APM_API_KEY;
       delete process.env.ELASTIC_APM_KIBANA_FRONTEND_ACTIVE;
       delete process.env.ELASTIC_APM_SERVER_URL;
-      delete process.env.ELASTIC_APM_GLOBAL_LABELS;
       delete process.env.NODE_ENV;
     });
 
@@ -186,21 +185,6 @@ describe('ApmConfiguration', () => {
           })
         );
       });
-
-      it('ELASTIC_APM_GLOBAL_LABELS', () => {
-        process.env.ELASTIC_APM_GLOBAL_LABELS = 'test1=1,test2=2';
-        const config = new ApmConfiguration(mockedRootDir, {}, true);
-
-        expect(config.getConfig('serviceName')).toEqual(
-          expect.objectContaining({
-            globalLabels: {
-              git_rev: 'sha',
-              test1: '1',
-              test2: '2',
-            },
-          })
-        );
-      });
     });
 
     it('ELASTIC_APM_KIBANA_FRONTEND_ACTIVE', () => {

packages/kbn-apm-config-loader/src/config.ts

@@ -7,7 +7,6 @@
  */
 
 import { join } from 'path';
-import deepmerge from 'deepmerge';
 import { merge, isEmpty } from 'lodash';
 import { execSync } from 'child_process';
 import { getDataPath } from '@kbn/utils';
@@ -310,10 +309,7 @@ export class ApmConfiguration {
     const { servicesOverrides, redactUsers, ...configFromKibanaConfig } =
       this.getConfigFromKibanaConfig();
     const configFromEnv = this.getConfigFromEnv(configFromKibanaConfig);
-    const config = [configFromKibanaConfig, configFromEnv].reduce<AgentConfigOptions>(
-      (acc, conf) => deepmerge(acc, conf),
-      {}
-    );
+    const config = merge({}, configFromKibanaConfig, configFromEnv);
 
     if (config.active === false && config.contextPropagationOnly !== false) {
       throw new Error(