Reuse string/InstanceDesc buffers in index gateway clients (#9341)

**What this PR does / why we need it**:

When running the IndexGateway in ring mode, `ring.MakeBuffersForGet()`
in the index gateway client allocated fixed size
`[]string`/`[]InstanceDesc` slices with a capacity of 5 for each and
every request.
By using a `sync.Pool` the client can reduce the amount of allocations.
Additionally, since we run the index gateway ring with a high
replication factor to be able to scale them better horizontally, an
initial capacity of 5 is most of the time too low, leading to extra
allocations when growing the capacity of the slice.

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

---------

Signed-off-by: Christian Haudum <christian.haudum@gmail.com>

pkg/storage/stores/indexshipper/gatewayclient/gateway_client.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"math/rand"
+	"sync"
 
 	"github.com/go-kit/log"
 	"github.com/go-kit/log/level"
@@ -97,6 +98,9 @@ type GatewayClient struct {
 	pool *ring_client.Pool
 
 	ring ring.ReadRing
+
+	stringBufPool   *sync.Pool
+	instanceBufPool *sync.Pool
 }
 
 // NewGatewayClient instantiates a new client used to communicate with an Index Gateway instance.
@@ -142,6 +146,20 @@ func NewGatewayClient(cfg IndexGatewayClientConfig, r prometheus.Registerer, log
 			return igPool, nil
 		}
 
+		// Replication factor plus additional room for JOINING/LEAVING instances
+		// See also ring.GetBufferSize
+		bufSize := cfg.Ring.ReplicationFactor() * 3 / 2
+		sgClient.stringBufPool = &sync.Pool{
+			New: func() any {
+				return make([]string, 0, bufSize)
+			},
+		}
+		sgClient.instanceBufPool = &sync.Pool{
+			New: func() any {
+				return make([]ring.InstanceDesc, 0, bufSize)
+			},
+		}
+
 		sgClient.pool = clientpool.NewPool(cfg.PoolConfig, sgClient.ring, factory, logger)
 	} else {
 		sgClient.conn, err = grpc.Dial(cfg.Address, dialOpts...)
@@ -321,10 +339,15 @@ func (s *GatewayClient) ringModeDo(ctx context.Context, callback func(client log
 		return errors.Wrap(err, "index gateway client get tenant ID")
 	}
 
-	bufDescs, bufHosts, bufZones := ring.MakeBuffersForGet()
+	bufDescs := s.instanceBufPool.Get().([]ring.InstanceDesc)
+	defer s.instanceBufPool.Put(bufDescs) //nolint:staticcheck
+	bufHosts := s.stringBufPool.Get().([]string)
+	defer s.stringBufPool.Put(bufHosts) //nolint:staticcheck
+	bufZones := s.stringBufPool.Get().([]string)
+	defer s.stringBufPool.Put(bufZones) //nolint:staticcheck
 
 	key := util.TokenFor(userID, "" /* labels */)
-	rs, err := s.ring.Get(key, ring.WriteNoExtend, bufDescs, bufHosts, bufZones)
+	rs, err := s.ring.Get(key, ring.WriteNoExtend, bufDescs[:0], bufHosts[:0], bufZones[:0])
 	if err != nil {
 		return errors.Wrap(err, "index gateway get ring")
 	}