[#240] logs: Move log messages to constants

Drop duplicate entities.
Format entities.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/services/session/executor.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/session"
+	"git.frostfs.info/TrueCloudLab/frostfs-node/internal/logs"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/util/logger"
 	"go.uber.org/zap"
 )
@@ -28,7 +29,7 @@ func NewExecutionService(exec ServiceExecutor, l *logger.Logger) Server {
 }
 
 func (s *executorSvc) Create(ctx context.Context, req *session.CreateRequest) (*session.CreateResponse, error) {
-	s.log.Debug("serving request...",
+	s.log.Debug(logs.SessionServingRequest,
 		zap.String("component", "SessionService"),
 		zap.String("request", "Create"),
 	)