Unable to change default port

[NathanD14]

After thoroughly checking the code, the port on which pm2-metrics runs could be changed in exporter.js in earlier versions (1.0.25). But in version 1.0.38 this does not work as it is hardcoded in package.json

[saikatharryc]

Hi, @NathanD14 I would love it if you can raise a PR maybe. Thank you!