model/modeldecoder: stop extracting "user.user-agent" from metadata fields

[axw]

In beater/middleware (in request_metadata_middleware.go) we record metadata fields related to the HTTP request received. When we process stream metadata, we first create a map based on the request metadata, and then layer on top the stream metadata.

One of the keys set by the middleware is user.user-agent, which is not part of the JSON Schema. In theory agents could send this field and the server would parse it.

I propose we have the middleware record information into a struct, and then when decoding stream metadata we initialise the metadata struct with those fields explicitly. i.e. something like

func UserMetadataMiddleware() Middleware {
        return func(h request.Handler) (request.Handler, error) {
                return func(c *request.Context) {
                        dec := utility.ManualDecoder{}
                        c.RequestMetadata.UserAgent = dec.UserAgentHeader(c.Request.Header)
                        c.RequestMetadata.User.IP = utility.ExtractIP(c.Request)
                        h(c)
                }, nil
        }
}

metadata := c.RequestMetadata
err := modeldecoder.DecodeMetadata(input, &metadata)
...

[axw]

Closed by #3943