Multiple ServerTiming instances should not share label(s)

[dotnetCarpenter]

I think it is confusing that each label has to be distinct across multiple instances of ServerTiming.

Steps to reproduce

'use strict'

const ServerTiming = require('servertiming')

function time (label, promise) {
	return new Promise ((resolve, reject) => {

		const timing = new ServerTiming()
		timing.startTimer (label)

		promise.then(value => {
			timing.stopTimer (label)
			resolve ([value, {"Server-Timing":timing.generateHeader()}])
		})

	})
}

function heavyComputation (value) {
	return new Promise ((resolve, reject) => {
		setTimeout (() => {
			resolve (value)
		}, 500)
	})
}

Promise.all([
	time('Multiple ServerTiming instances should not share labels', heavyComputation(1)),
	time('Multiple ServerTiming instances should not share labels', heavyComputation(2)),
	time('Multiple ServerTiming instances should not share labels', heavyComputation(3)),
])
.then(timings => {
	timings.map(([value, header]) => console.log(value, header['Server-Timing']))
})
.catch (console.error)

The above will output:

1 multiple-servertiming-instances-should-not-share-labels; dur=500.8789; desc="Multiple ServerTiming instances should not share labels"
2 multiple-servertiming-instances-should-not-share-labels; dur=0; desc="Multiple ServerTiming instances should not share labels"
3 multiple-servertiming-instances-should-not-share-labels; dur=0; desc="Multiple ServerTiming instances should not share labels"

Obviously the duration is not correct.

node-servertiming: 1.5.0
node: v14.16.1