Ensure our proxy helpers forward metadata

[cretz]

Describe the solution you'd like

This would be an issue for https://pkg.go.dev/go.temporal.io/api/proxy but there is no "issues" section there. Basically we need to confirm that when using the proxy library in the default way, that the headers like client-name/client-version flow through the proxy. Write a test and, if it's not flowing through, make it do so. This may just be a case of our code gen using https://pkg.go.dev/google.golang.org/grpc/metadata to populating incoming context to outgoing context (and adding an option to disable it). Probably can't have an interceptor since users control that part, we just control the service proxy helper.

[cretz]

PR opened at temporalio/api-go#167 (which will be waiting for #1502 to be released before merging).