Add support for openmetrics 0.0.1 besides 1.0.0

[rg2011]

Continuation to #1627 - The current version of prometheus collector in openshift has been found to request openmetrics 0.0.1

[rg2011]

I changed the subject to [WIP] because I intend to add some unit tests of the openmetricsHandler function to make sure Accept headers are processed correctly. But the tests would require a new devDependency to node-mocks-http

@fgalan , @AlvaroVega, would you be ok with adding devDependency node-mocks-http?

[AlvaroVega]

@fgalan , @AlvaroVega, would you be ok with adding devDependency node-mocks-http?

Yes, I'm OK with that.

[rg2011]

finally I refactored the code I wanted to test out of the request handler, so I could unit test it without dependencies. The PR i sready for review.

[fgalan]

In order to avoid magic numbers maybe it's better this way:

Suggested change

	for (let part of parts.slice(1)) {
	if (part.startsWith('version=')) {
	version = part.substring(8).trim();
	} else if (part.startsWith('charset=')) {
	charset = part.substring(8).trim();
	} else if (part.startsWith('q=')) {
	preference = parseFloat(part.substring(2).trim());
	let versionTokenLength = 'version='.length;
	let qTokenLength = 'q='.length;
	for (let part of parts.slice(1)) {
	if (part.startsWith('version=')) {
	version = part.substring(versionTokenLength ).trim();
	} else if (part.startsWith('charset=')) {
	charset = part.substring(versionTokenLength ).trim();
	} else if (part.startsWith('q=')) {
	preference = parseFloat(part.substring(qTokenLength).trim());

(Code not actually tested, it may not run, but you get the idea :)

[rg2011]

fix in 679813e

[fgalan]

LGTM

Passing the ball to @AlvaroVega for extra review and LGTM

[AlvaroVega]

LGTM