[opentelemetry-exporter-prometheus] Export PrometheusSerializer

[matschaffer]

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

I would like to implement a PrometheusExporter that fits into the kibana routing infrastrucure. The implementation of the exporter is simple if I can reuse the PrometheusSerializer, but it's not exported from the package index.

Describe the solution you'd like

Exporting it should fix this, if it's acceptable to have as part of the public API.

Describe alternatives you've considered

For now I've cloned a slightly-modified version of the PrometheusSerializer code into my own kibana PR.

elastic/kibana@05389c3 (#133171)

[weyert]

Can’t you use the request handler that’s available on PrometheusExporter?
That’s what I am using the expose a a metrics a endpoint on my services to avoid a second http server

I am doing exporter. getMetricsRequestHandler(req, res)

[matschaffer]

See #3034 (comment) for followup on above comment.