Allow trends to be recorded as histograms instead of summaries via options argument

[ahinh43]

Closes #17

This PR adds the ability to optionally record trend metrics as Prometheus Histograms instead of Summary types. This is controlled with a new option argument named -o usehistogramfortime=yes (name suggestions are welcome 😅)

This can be tested by passing the argument into the K6 binary when calling it after build:

Command:

./k6 run -o 'prometheus=usehistogramfortime=yes' script.js

Which then the metrics endpoint will output this example in the trend related metrics

# TYPE http_req_duration histogram
...
http_req_duration_bucket{expected_response="true",group="",method="GET",name="http://test.k6.io",proto="HTTP/1.1",scenario="default",status="301",tls_version="",url="http://test.k6.io",le="2.5"} 0
http_req_duration_bucket{expected_response="true",group="",method="GET",name="http://test.k6.io",proto="HTTP/1.1",scenario="default",status="301",tls_version="",url="http://test.k6.io",le="5"} 0
...
http_req_duration_bucket{expected_response="true",group="",method="GET",name="http://test.k6.io",proto="HTTP/1.1",scenario="default",status="301",tls_version="",url="http://test.k6.io",le="+Inf"} 2

If you don't pass usehistogramfortime=yes then the default summary type is still used.

Command:

./k6 run -o prometheus script.js

Output:

# TYPE http_req_duration summary
http_req_duration{expected_response="true",group="",method="GET",name="http://test.k6.io",proto="HTTP/1.1",scenario="default",status="301",tls_version="",url="http://test.k6.io",quantile="0.5"} 31.079
http_req_duration{expected_response="true",group="",method="GET",name="http://test.k6.io",proto="HTTP/1.1",scenario="default",status="301",tls_version="",url="http://test.k6.io",quantile="0.9"} 32.001
http_req_duration{expected_response="true",group="",method="GET",name="http://test.k6.io",proto="HTTP/1.1",scenario="default",status="301",tls_version="",url="http://test.k6.io",quantile="0.95"} 32.001
http_req_duration{expected_response="true",group="",method="GET",name="http://test.k6.io",proto="HTTP/1.1",scenario="default",status="301",tls_version="",url="http://test.k6.io",quantile="1"} 32.001

I've updated the README where I thought would be necessary but if there's any other places that should be updated please let me know. Thanks!