[Fleet] Fix merge config with APM change (#3508) (#3509)

(cherry picked from commit 3fbba14)

Co-authored-by: Nicolas Chaulet <nicolas.chaulet@elastic.co>

internal/pkg/config/config.go

@@ -26,6 +26,13 @@ var DefaultOptions = []ucfg.Option{
 	ucfg.FieldReplaceValues("inputs"),
 }
 
+var MergeOptions = []ucfg.Option{
+	ucfg.PathSep("."),
+	ucfg.ResolveEnv,
+	ucfg.VarExp,
+	ucfg.FieldMergeValues("inputs"),
+}
+
 const kRedacted = "[redacted]"
 
 // Config is the global configuration.

internal/pkg/server/agent.go

@@ -433,7 +433,7 @@ func (a *Agent) configFromUnits(ctx context.Context) (*config.Config, error) {
 					},
 				},
 				}}
-			err = cfgData.Merge(obj, config.DefaultOptions...)
+			err = cfgData.Merge(obj, config.MergeOptions...)
 			if err != nil {
 				zerolog.Ctx(ctx).Warn().Err(err).Msg("Failed to merge APM config into cfgData")
 			}

internal/pkg/server/agent_test.go

@@ -253,6 +253,9 @@ func Test_Agent_configFromUnits(t *testing.T) {
 			"server": map[string]interface{}{
 				"host": "0.0.0.0",
 			},
+			"policy": map[string]interface{}{
+				"id": "test-policy",
+			},
 		})
 		require.NoError(t, err)
 		mockInClient := &mockClientUnit{}
@@ -276,8 +279,18 @@ func Test_Agent_configFromUnits(t *testing.T) {
 				},
 			})
 
+		cliCfg, err := ucfg.NewFrom(map[string]interface{}{
+			"inputs": []interface{}{
+				map[string]interface{}{
+					"policy": map[string]interface{}{
+						"id": "test-policy",
+					},
+				},
+			},
+		})
+		require.NoError(t, err)
 		a := &Agent{
-			cliCfg:     ucfg.New(),
+			cliCfg:     cliCfg,
 			agent:      mockAgent,
 			inputUnit:  mockInClient,
 			outputUnit: mockOutClient,
@@ -287,6 +300,7 @@ func Test_Agent_configFromUnits(t *testing.T) {
 		require.NoError(t, err)
 		require.Len(t, cfg.Inputs, 1)
 		assert.Equal(t, "fleet-server", cfg.Inputs[0].Type)
+		assert.Equal(t, "test-policy", cfg.Inputs[0].Policy.ID)
 		assert.Equal(t, "0.0.0.0", cfg.Inputs[0].Server.Host)
 		assert.True(t, cfg.Inputs[0].Server.Instrumentation.Enabled)
 		assert.False(t, cfg.Inputs[0].Server.Instrumentation.TLS.SkipVerify)