Fix Content-Type header for Prometheus compliance

[Copilot]

The PM2 Prometheus exporter was sending an incomplete Content-Type header that caused Prometheus to report scraping errors:

Error scraping target: non-compliant scrape target sending blank Content-Type and no fallback_scrape_protocol specified for target

While a Content-Type header was being set, it was missing the required charset=utf-8 parameter that Prometheus expects according to the exposition format specification.

Before:

Content-Type: text/plain; version=0.0.4

After:

Content-Type: text/plain; version=0.0.4; charset=utf-8

This change ensures full compliance with the Prometheus exposition format and eliminates the need for users to configure workarounds like fallback_scrape_protocol: "PrometheusText0.0.4" in their Prometheus scrape configs.

The fix is minimal and surgical - only adding the missing charset parameter to the existing Content-Type header without affecting any other functionality.

Fixes #101.

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[ths83]

@saikatharryc Is it possible to merge this MR soon ?

[ths83]

@saikatharryc Thanks for opening the MR! Should you also merge it manually?

[saikatharryc]

@ths83 - i realized that i did not maintain it for a while and even if i merge this it wont release automatically to the npm.

[aroy314]

can we merge and release please ?
in the meantime if you need your prometheus to work without it, add fallback_scrape_protocol in your config :

prometheus['scrape_configs'] = [
	{
	 'job_name': 'node',
	 'fallback_scrape_protocol' => 'PrometheusText0.0.4', # for pm2 content-type header
	 'static_configs' => [
	   'targets' => ['domain.com:9209'],
	 ],
	}
]