Update to use latest prom-client

[fksimon]

The latest prom-client 13.0.x does not work due to some api changes. The new prom-client has some substantial enhancements so would be nice to be able to use.

https://github.com/siimon/prom-client/releases

Main impacting change:
metrics server has a line like res.send(register.metrics()), you
should change it to res.send(await register.metrics()).

[yorch]

Created a PR #65

[robross0606]

prom-client v14 has now been released.

[paulo-dc]

Hello, prom-client v15 has now been released. (last version : 15.1.3)

But the version 3.2.2 of prometheus-api-metrics require peer depedency prom-client@">=12 <15" . While the master branch has peer depedency "prom-client": ">=12 <16".

Do you plan to deliver new release soon ?