[#569] cli: Add control shards writecache seal command

It does the same as `control shards flush-writecache --seal`, but
has better name.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/writecache/seal.go

@@ -0,0 +1,28 @@
+package writecache
+
+import (
+	"context"
+
+	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/shard/mode"
+	"git.frostfs.info/TrueCloudLab/frostfs-observability/tracing"
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/trace"
+)
+
+func (c *cache) Seal(ctx context.Context, ignoreErrors bool) error {
+	ctx, span := tracing.StartSpanFromContext(ctx, "writecache.Seal",
+		trace.WithAttributes(
+			attribute.Bool("ignore_errors", ignoreErrors),
+		))
+	defer span.End()
+
+	c.modeMtx.Lock()
+	defer c.modeMtx.Unlock()
+
+	// flush will be done by setMode
+	err := c.setMode(ctx, mode.DegradedReadOnly, ignoreErrors)
+	if err == nil {
+		c.metrics.SetMode(mode.DegradedReadOnly)
+	}
+	return err
+}