Fix gRPC reflection for local services

[oscar-meroton]

Commit 5b5db75,
"Add generic gRPC stream forwarding (#306)",
correctly added forwarding of grpc_reflection_v1 to configured backends. Unfortunately, it stopped the local services from being reported.

[EdSchouten]

@moroten PTAL!

[aspect-workflows]

Test

27 test targets passed

Targets

//pkg/auth:auth_test [k8-fastbuild]                                                           868ms
//pkg/blobstore/buffer:buffer_test [k8-fastbuild]                                             66ms
//pkg/blobstore/completenesschecking:completenesschecking_test [k8-fastbuild]                 61ms
//pkg/blobstore/grpcclients:grpcclients_test [k8-fastbuild]                                   152ms
//pkg/blobstore/grpcservers:grpcservers_test [k8-fastbuild]                                   246ms
//pkg/blobstore/local:local_test [k8-fastbuild]                                               107ms
//pkg/blobstore/mirrored:mirrored_test [k8-fastbuild]                                         150ms
//pkg/blobstore/readcaching:readcaching_test [k8-fastbuild]                                   160ms
//pkg/blobstore/readfallback:readfallback_test [k8-fastbuild]                                 52ms
//pkg/blobstore/replication:replication_test [k8-fastbuild]                                   183ms
//pkg/blobstore/sharding/integration:integration_test [k8-fastbuild]                          90ms
//pkg/blobstore/sharding:sharding_test [k8-fastbuild]                                         659ms
//pkg/blobstore:blobstore_test [k8-fastbuild]                                                 56ms
//pkg/blockdevice:blockdevice_test [k8-fastbuild]                                             190ms
//pkg/builder:builder_test [k8-fastbuild]                                                     67ms
//pkg/capabilities:capabilities_test [k8-fastbuild]                                           161ms
//pkg/digest:digest_test [k8-fastbuild]                                                       76ms
//pkg/filesystem/path:path_test [k8-fastbuild]                                                75ms
//pkg/filesystem:filesystem_test [k8-fastbuild]                                               137ms
//pkg/grpc:grpc_test [k8-fastbuild]                                                           70ms
//pkg/http/server:server_test [k8-fastbuild]                                                  44ms
//pkg/jmespath:jmespath_test [k8-fastbuild]                                                   361ms
//pkg/jwt:jwt_test [k8-fastbuild]                                                             216ms
//pkg/otel:otel_test [k8-fastbuild]                                                           56ms
//pkg/prometheus:prometheus_test [k8-fastbuild]                                               53ms
//pkg/util:util_test [k8-fastbuild]                                                           88ms
//pkg/x509:x509_test [k8-fastbuild]                                                           43ms

Total test execution time was 4s. 3 tests (10.0%) were fully cached saving 297ms.

[EdSchouten]

Acked by @stagnation via Slack.