caddyhttp: Make metrics opt-in

[mholt]

Related to #4644

Metrics have accumulated an unfortunate performance regression and have some functionality limitations. Since we don't have the time or expertise on hand currently to address these issues in the more ideal way, I think we need to make metrics opt-in for now.

In the JSON, metrics are enabled by setting "metrics": {} in your server config, or in the Caddyfile, use global options:

{
	servers {
		metrics
	}
}

This is EXPERIMENTAL and subject to change.

Note that metrics for handle_response handlers (etc) will not be able to have metrics enabled, at least for now.

I think going forward a better long-term fix might be to have the metrics directive enable metrics, rather than a server-wide config. I'd also like to make certain aspects about metrics actually configurable, rather than having an empty struct.

[hairyhenderson]

Worth it until we can get the perf issues sorted out!

[francislavoie]

I think I agree with this as well, as unfortunate as it is.

I guess we only end up with like admin metrics if this is off? What does the metrics endpoint end up showing? Will it just show "no requests" for the vectors it tracks? That could be confusing/surprising to people who upgrade.

[mholt]

Yeah, I'll make it very clear in the release notes that metrics are opt-in now.