Prevent Access to "grpc-" headers

Headers that begin with "grpc-" are meant for use by gRPC and not by other clients. Using this as a namespace allows the gRPC team to add new headers without fear of colliding with another users headers.

To this end, such headers should be locked down from reading and writing in the library.

Start by tracing how the Java library reads and writes metadata headers, then identify the validation or access-control entry points for reserved names. Check the existing metadata tests before defining coverage for headers beginning with "grpc-". Done means those headers cannot be read or written by other clients while existing header behavior remains covered.

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