MM-11029 Adding plugin logging functionality. (#9034)

* Capturing stdout, stderr of plugins in logs.

* Cleanup go-plugin debug logs.

* Adding logging to plugin API

* Generating mocks.

* godoc convention

plugin/hclog_adapter.go

@@ -6,6 +6,7 @@ package plugin
 import (
 	"fmt"
 	"log"
+	"strings"
 
 	"github.com/hashicorp/go-hclog"
 	"github.com/mattermost/mattermost-server/mlog"
@@ -17,23 +18,48 @@ type HclogAdapter struct {
 }
 
 func (h *HclogAdapter) Trace(msg string, args ...interface{}) {
-	h.wrappedLogger.Debug(msg, mlog.String(h.extrasKey, fmt.Sprintln(args...)))
+	extras := strings.TrimSpace(fmt.Sprint(args...))
+	if extras != "" {
+		h.wrappedLogger.Debug(msg, mlog.String(h.extrasKey, extras))
+	} else {
+		h.wrappedLogger.Debug(msg)
+	}
 }
 
 func (h *HclogAdapter) Debug(msg string, args ...interface{}) {
-	h.wrappedLogger.Debug(msg, mlog.String(h.extrasKey, fmt.Sprintln(args...)))
+	extras := strings.TrimSpace(fmt.Sprint(args...))
+	if extras != "" {
+		h.wrappedLogger.Debug(msg, mlog.String(h.extrasKey, extras))
+	} else {
+		h.wrappedLogger.Debug(msg)
+	}
 }
 
 func (h *HclogAdapter) Info(msg string, args ...interface{}) {
-	h.wrappedLogger.Info(msg, mlog.String(h.extrasKey, fmt.Sprintln(args...)))
+	extras := strings.TrimSpace(fmt.Sprint(args...))
+	if extras != "" {
+		h.wrappedLogger.Info(msg, mlog.String(h.extrasKey, extras))
+	} else {
+		h.wrappedLogger.Info(msg)
+	}
 }
 
 func (h *HclogAdapter) Warn(msg string, args ...interface{}) {
-	h.wrappedLogger.Warn(msg, mlog.String(h.extrasKey, fmt.Sprintln(args...)))
+	extras := strings.TrimSpace(fmt.Sprint(args...))
+	if extras != "" {
+		h.wrappedLogger.Warn(msg, mlog.String(h.extrasKey, extras))
+	} else {
+		h.wrappedLogger.Warn(msg)
+	}
 }
 
 func (h *HclogAdapter) Error(msg string, args ...interface{}) {
-	h.wrappedLogger.Error(msg, mlog.String(h.extrasKey, fmt.Sprintln(args...)))
+	extras := strings.TrimSpace(fmt.Sprint(args...))
+	if extras != "" {
+		h.wrappedLogger.Error(msg, mlog.String(h.extrasKey, extras))
+	} else {
+		h.wrappedLogger.Error(msg)
+	}
 }
 
 func (h *HclogAdapter) IsTrace() bool {