Improve guard to avoid invalid number of arguments error message

[leftieFriele]

Description of the problem

There's a type of error which happens frequently in applications using prom-client where the arguments don't match up.
It could be that a label on a metrics is derived from a variable and in certain cases this variable is undefined, leading to the error message seen here:

  Error: Invalid number of arguments
    at exports.getLabels (/app/node_modules/prom-client/lib/util.js:56:9)
    at Histogram.labels (/app/node_modules/prom-client/lib/histogram.js:164:18)

Proposed solutions

• send a pull request to the prom-client project with an error message which includes label names & args so it's easier to debug this line
• improve the guard to check for labels being undefined or null