feat: METRICS env var should enable libp2p promclient metrics?

[SgtPooki]

helia-http-gateway/src/constants.ts

Lines 7 to 12 in bce0558

	/**
	* If set to any value other than 'true', we will disable prometheus metrics.
	*
	* @default 'true'
	*/
	export const METRICS = process.env.METRICS ?? 'true'

we may want a separate env var for this

came up during https://pl-strflt.notion.site/2023-10-26-Helia-Working-Group-ac880b0ddb244f64bb224367167e2ad7?pvs=4