Sampling more streams to measure error rate correctly

[masaori335]

"Closing a H2 connection with high stream error rate" is added by 693b3cb.
The close event could be triggered when the first stream got error regardless value of proxy.config.http2.stream_error_rate_threshold. Because the error rate bacomes 1. We should wait for more streams to measure stream error rate correctly.

trafficserver/proxy/http2/Http2ClientSession.cc

Lines 383 to 384 in 9b73fc9

	} else if (this->connection_state.get_stream_error_rate() >
	Http2::stream_error_rate_threshold) { // For a case many stream errors happened

trafficserver/proxy/http2/Http2ConnectionState.h

Lines 224 to 233 in 9b73fc9

	double
	get_stream_error_rate() const
	{
	int total = get_stream_requests();
	if (total > 0) {
	return (double)stream_error_count / (double)total;
	} else {
	return 0;
	}
	}

[maskit]

We should wait for more streams to measure stream error rate correctly.

Why? If a client sent 1 request and it failed, the error rate is obviously 100%.

A error rate won't be less than 10% until a client makes 10 requests if a stream error happened on the first stream, and it would be 20 if the threshold was 5%.

Do we want more requests from a client that cannot do things correctly on the first stream?

[masaori335]

Fixed by #6525