Cache ManagedChannel and shared call context in JsonToGrpcGatewayFilterFactory (fixes leak, ~2.6× throughput)

Is your feature request related to a problem? Please describe.

Under load, JsonToGrpcGatewayFilterFactory leaks gRPC ManagedChannel instances and repeats expensive initialization on every request.

A 400-request / 50-concurrency load test against a route that uses this filter produces 182 the errors below:

ERROR i.g.i.ManagedChannelOrphanWrapper > cleanQueue :
  *~*~*~ Previous channel ManagedChannelImpl{logId=7, target=localhost:6565}
  was not shutdown properly!!! ~*~*~*
java.lang.RuntimeException: ManagedChannel allocation site

There are two underlying problems in the current factory:

1. ManagedChannel is created per request and never shut down.

• createChannelChannel(host, port) is called from inside GRPCResponseDecorator for every exchange.

2. Heavy per-route initialization runs per request.

• GRPCResponseDecorator is constructed for every exchange, and its constructor parses the proto descriptor file, builds the FileDescriptor graph, builds MethodDescriptor + marshallers, and stuffs — all of which only need to be built once per route.

Describe the solution you'd like

Two changes to JsonToGrpcGatewayFilterFactory:

1. Cache ManagedChannel per host:port on the factory. Move the cache to a ConcurrentHashMap<String, ManagedChannel> field on JsonToGrpcGatewayFilterFactory.

2. Extract a GrpcCallContext that holds the parsed proto descriptors, the MethodDescriptor, the shared JsonFormat.Parser / Printer, the ObjectMapper, and the empty ObjectNode. Build it once in apply(Config) and reuse it for every request on that route.

Measured impact with the same hey load (400 req / 50 concurrency):

Metric	Before	After	Change
Requests/sec	688.66	1805.41	+162%
Average latency	62.4 ms	23.3 ms	−63%
p50	47.7 ms	18.7 ms	−61%
p95	145.7 ms	57.6 ms	−60%
p99	175.9 ms	86.1 ms	−51%
Channel-leak errors	182	0	—

Describe alternatives you've considered

N/A

Additional context

• Related issue: #3120. PR #3122 was approved in April 2024 but has been unmerged for over a year, and as noted above does not actually resolve the leak.
• I'm happy to open a PR with the changes above if this direction is acceptable.

Environment

• Spring Boot 3.4.5
• Spring Cloud 2024.0.1 (Moorgate)
• Spring Cloud Gateway 4.2.x
• grpc-java 1.69.0
• Netty 4.1.130.Final
• Java 17