Access information about the request and response payloads in grpc-go's stat/HandleRPC

I am using stats/HandleRPC() to emit some metrics about the RPC duration, when I receive the stats/End data, and I want to tag the metrics with some information that can be extracted from the incoming and outgoing payloads. What would be the best way to achieve this?

func (h *myStatsHandler) HandleRPC(ctx context.Context, rpcStats stats.RPCStats) {
    switch stat := rpcStats.(type) {
    case *stats.End:
        durationMs := stat.EndTime.Sub(stat.BeginTime).Seconds() * 1000.0
        // Now before sending this value, I need to know, for example the value of a specific key in the request payload, or whether the response is nil or not 
    }
}