Add stopwatch info into response headers

[joelwurtz]

Q	A
Bug?	no
New Feature?	yes

It's very hard actually to know from which request a stop watch event comes from, specially when you do multiples request over the same endpoint (e.g. GraphQL)

I'm proposing adding information from stopwatch : time / memory directly into responses headers so it will be easier to reuse that information (AFAIK there is no other possibility with PSR7), so a response could have :

• X-StopWatch-Time: 6543213 (in micro seconds)
• X-StopWatch-Memory: 32132465 (in bytes)