[Lua workaround] gRPC dissector does not start Protobuf sub-dissector when capture is incomplete

[Alexander Petrossian <paf () yandex ru>]

Friends, currently when gRPC plugin fails to get content-type header value it stops and does not invoke Protobuf 
dissector, which makes me sad:

    http2_content_type = http2_get_header_value(pinfo, HTTP2_HEADER_CONTENT_TYPE, FALSE);
    if (http2_content_type == NULL || http2_path == NULL) {
        return; /* not continue if there is not enough grpc information */
    }

Thing is, traces are more often recorded without start of communication.
And even when gRPC body is there, content-type field was compressed and can not be decompressed = 
http2_get_header_value returns NULL.

Could one do some workaround in Lua right now?
Somehow force gRPC hand to invoke Protobuf dissector even without content-type: application/grpc.

I’m thinking of wedging between dissectors and provide some default value of content-type or some such. But that 
approach seems vague. Any ideas on it or other?

Thanks in advance!
PAF

P.S. I’ve suggested a seemingly trivial untested workaround in C here
https://gitlab.com/wireshark/wireshark/-/issues/17793

Right now I’m interested in some Lua approach that we could employ without recompilings...
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