h2c-client.go

client := http.Client{
    Transport: &http2.Transport{
        // So http2.Transport doesn't complain the URL scheme isn't 'https'
        AllowHTTP: true,
        // Pretend we are dialing a TLS endpoint.
        // Note, we ignore the passed tls.Config
        DialTLSContext: func(ctx context.Context, n, a string, _ *tls.Config) (net.Conn, error) {
            var d net.Dialer
            return d.DialContext(ctx, n, a)
        },
    },
}

resp, _ := client.Get(url)
fmt.Printf("Client Proto: %d\n", resp.ProtoMajor)