Expose request handler of Prometheus exporter

[weyert]

Is your feature request related to a problem? Please describe.

I would like to reuse the request handler that's implemented by the Prometheus exporter so that it can be used by different server frameworks in Node.js

Describe the solution you'd like

If the currently private request handler method is exposed by the Prometheus exporter it can be used by a wide variety of server framework in Node.js, such as Express, Koa, Hapi etc.

If you want to expose the endpoint using Express you could do something like:

server.get('/metrics', (req, res) => {
    return exporter.getRequestHandler(req, res)
  })

Alternatively, if you want to expose the endpoint using Koa:

 router.get('/metrics', async (ctx) => {
    await exporter.getRequestHandler(ctx.req, ctx.res)
    ctx.respond = false
  })

Describe alternatives you've considered

N/A

Additional context

Allow to use an existing endpoint to return Prometheus metrics based on the opentelemetry prometheus exporter implementation.

[vmarchaud]

For more information, someone already tried to work on this here: #1701 however we agreed on a different way of solving the issue (pretty much what you suggested) instead of what the original PR was doing.

Are you interested in contributing this @weyert or you should we add a label to say that its open for anyone to contribute ?

[weyert]

@vmarchaud Sure, if you like my suggestion, I would be happy to work on a PR to apply the suggestion. Might take a bit of time as I need to get used to the code base a bit more :)

[vmarchaud]

@weyert Then i'll assign it to you, feel free to ping on gitter or open a draft PR if you want some help

[weyert]

@vmarchaud I have started working on the pull request. This library is using mocha/chai and I don't have real experience with these testing libraries mainly jest. I will come back to the unit test once I found their version of jest.fn(). Anyways this is the approach I am thinking off.

[vmarchaud]

@weyert We generally use sinon for this, here is an example: https://github.com/open-telemetry/opentelemetry-js/blob/main/packages/opentelemetry-exporter-collector/test/node/CollectorTraceExporter.test.ts#L54