fix: negative trace substraction when using SetTimeout

[nicolasbeauvais]

In certain cases when using client or request SetTimeout it is possible that some trace value like dnsDone, tlsHandshakeDone or gotConn are zero, resulting in inaccurate results:

This PR add additional checks in the TraceInfo method to avoid negative values.

I'm not sure how to improve the tests for this, as aborting requests with SetTimeout will not always accurately stop the execution at the exact same point. I'm open to ideas.

[jeevatkm]

@nicolasbeauvais Thanks for the PR. I'm sorry for the delayed response, I was occupied with my personal stuff.

I added a one review comment.

[jeevatkm]

@nicolasbeauvais - I would suggest changing the code flow as follows:

ti.DNSLookup = 0
if !ct.dnsStart.IsZero() && !ct.dnsDone.IsZero() {
	ti.DNSLookup = ct.dnsDone.Sub(ct.dnsStart)
}

ti.TLSHandshake = 0
if !ct.tlsHandshakeDone.IsZero() && !ct.tlsHandshakeStart.IsZero() {
	ti.TLSHandshake = ct.tlsHandshakeDone.Sub(ct.tlsHandshakeStart)
}

ti.ServerTime = 0
if !ct.gotFirstResponseByte.IsZero() && !ct.gotConn.IsZero() {
	ti.ServerTime = ct.gotFirstResponseByte.Sub(ct.gotConn)
}

[jeevatkm]

Also, the same issue might exist in the v2 too. I will backport it afterwards, or you can also submit PR v2 if you're interested.