fix(metricprovider): respect --logformat json for metric plugin process logger

The metric provider plugin client (metricproviders/plugin/client/client.go)
constructed its go-plugin ClientConfig without setting Logger, so go-plugin
always fell back to its own hardcoded unstructured-text hclog logger for the
plugin's handshake/lifecycle logs and stderr relay - regardless of the
controller's own --logformat setting. When the controller is run with
--logformat json, every other component logs JSON except metric plugin logs,
which stay plain text and can't be parsed the same way downstream.

Add newPluginLogger(), which mirrors the controller's own logrus formatter:
when the standard logger is configured with logrus.JSONFormatter, build an
hclog.Logger with JSONFormat: true (keeping go-plugin's other defaults -
Trace level, "plugin" name, DefaultOutput - unchanged); otherwise return nil
so go-plugin's own default logger is used exactly as before, preserving
existing behavior for the common (non-JSON) case.

Fixes #4408

Signed-off-by: Sujan Reddy <sujanchalla0510@gmail.com>

Author: sujanchalla0510

go.mod

@@ -19,6 +19,7 @@ require (
 	github.com/golang/protobuf v1.5.4
 	github.com/google/uuid v1.6.0
 	github.com/grpc-ecosystem/grpc-gateway v1.16.0
+	github.com/hashicorp/go-hclog v1.6.3
 	github.com/hashicorp/go-plugin v1.8.0
 	github.com/influxdata/influxdb-client-go/v2 v2.14.0
 	github.com/juju/ansiterm v1.0.0
@@ -125,7 +126,6 @@ require (
 	github.com/gregdel/pushover v1.3.1 // indirect
 	github.com/gregjones/httpcache v0.0.0-20190611155906-901d90724c79 // indirect
 	github.com/hashicorp/go-cleanhttp v0.5.2 // indirect
-	github.com/hashicorp/go-hclog v1.6.3 // indirect
 	github.com/hashicorp/go-retryablehttp v0.7.7 // indirect
 	github.com/hashicorp/yamux v0.1.2 // indirect
 	github.com/huandu/xstrings v1.5.0 // indirect

metricproviders/plugin/client/client.go

@@ -2,10 +2,13 @@ package client
 
 import (
 	"fmt"
+	"io"
 	"os/exec"
 	"sync"
 
+	"github.com/hashicorp/go-hclog"
 	goPlugin "github.com/hashicorp/go-plugin"
+	log "github.com/sirupsen/logrus"
 
 	"github.com/argoproj/argo-rollouts/metricproviders/plugin/rpc"
 	"github.com/argoproj/argo-rollouts/pkg/apis/rollouts/v1alpha1"
@@ -68,6 +71,7 @@ func (m *metricPlugin) startPluginSystem(metric v1alpha1.Metric) (rpc.MetricProv
 				Plugins:         pluginMap,
 				Cmd:             exec.Command(pluginPath, args...),
 				Managed:         true,
+				Logger:          newPluginLogger(),
 			})
 
 			rpcClient, err := m.pluginClient[pluginName].Client()
@@ -108,3 +112,27 @@ func (m *metricPlugin) startPluginSystem(metric v1alpha1.Metric) (rpc.MetricProv
 
 	return nil, fmt.Errorf("no plugin found")
 }
+
+// newPluginLogger builds the hclog.Logger used for the go-plugin client's own log output
+// (handshake/lifecycle logs, and the plugin process's stderr relay). By default go-plugin
+// falls back to its own unstructured text logger whenever ClientConfig.Logger is nil, which
+// is inconsistent with the controller's own logs when the controller is run with
+// `--logformat json`: every other component's logs are JSON, but metric plugin logs remain
+// plain text. When the controller's standard logger is configured for JSON output, mirror
+// that here so the plugin logger's output is JSON too. Returns nil (go-plugin's own default
+// logger) in every other case, preserving prior behavior exactly.
+func newPluginLogger() hclog.Logger {
+	return newPluginLoggerWithOutput(hclog.DefaultOutput)
+}
+
+func newPluginLoggerWithOutput(w io.Writer) hclog.Logger {
+	if _, ok := log.StandardLogger().Formatter.(*log.JSONFormatter); !ok {
+		return nil
+	}
+	return hclog.New(&hclog.LoggerOptions{
+		Output:     w,
+		Level:      hclog.Trace,
+		Name:       "plugin",
+		JSONFormat: true,
+	})
+}

metricproviders/plugin/client/client_test.go

@@ -0,0 +1,41 @@
+package client
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+
+	log "github.com/sirupsen/logrus"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestNewPluginLoggerRespectsControllerLogFormat verifies that the go-plugin client's own
+// logger mirrors the controller's configured log format. Before this fix, the plugin
+// client's Logger field was always left unset, so go-plugin always fell back to its
+// unstructured, hardcoded-text default logger regardless of the controller's own
+// `--logformat json` setting.
+func TestNewPluginLoggerRespectsControllerLogFormat(t *testing.T) {
+	origFormatter := log.StandardLogger().Formatter
+	defer log.StandardLogger().SetFormatter(origFormatter)
+
+	t.Run("default text logformat leaves go-plugin's own default logger untouched", func(t *testing.T) {
+		log.StandardLogger().SetFormatter(&log.TextFormatter{})
+		logger := newPluginLogger()
+		assert.Nil(t, logger, "expected nil so go-plugin falls back to its own default logger")
+	})
+
+	t.Run("json logformat produces a JSON plugin logger", func(t *testing.T) {
+		log.StandardLogger().SetFormatter(&log.JSONFormatter{})
+
+		var buf bytes.Buffer
+		logger := newPluginLoggerWithOutput(&buf)
+		assert.NotNil(t, logger)
+
+		logger.Info("plugin log line", "plugin", "my-plugin")
+
+		var parsed map[string]any
+		err := json.Unmarshal(buf.Bytes(), &parsed)
+		assert.NoError(t, err, "expected the plugin logger to emit a single JSON object, got: %s", buf.String())
+		assert.Equal(t, "plugin log line", parsed["@message"])
+	})
+}