Queue time mismatch between X-Request-Time scale and start time call

[cschneid]

The X-Request-Time header is in milliseconds, but the PHP code then does a divide by 10000, and uses microtime to compare against it, which I don't think matches up.

The current code:

• The header value comes in as milliseconds since epoch. To simplify, assume we're in 1970 and say "2".
• The code divides that number by 10000, so it's 0.0002
• Then it gets the start time as microseconds, say 3000 (3 milliseconds after epoch)
• The final value is then (3000 - 0.0002) * 1e9 = 2.999e12 nanoseconds. Which is ~50 minutes.

The expected value is:

• Header value of 2ms after epoch
• start time is 3ms after epoch
• start - header = 1ms
• convert to nanoseconds = 1e6 nanoseconds

Because of the subtraction, epoch cancels out.

https://github.com/scoutapp/scout-apm-php/blob/master/src/Events/Request/Request.php#L95-L96

https://devcenter.heroku.com/articles/http-routing#heroku-headers