[configgrpc] Default to TCP transport

[evan-bradley]

Component(s)

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Describe the issue you're reporting

Currently, it is required by components using configgrpc to set the transport option in their default config if they want to offer a default for this option. The godoc for configgrpc states that only tcp and unix are supported.

As a networked application, I would assume it's relatively uncommon for Collector users to opt for gRPC over Unix sockets vs. over a TCP connection. I think we should therefore consider making the default configgrpc transport type TCP (which is what many components already do).

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