docs: clarify the behavior of prepending hostname to metrics

[huikang]

Description

The prepended name to the gauge metrics causes some confusion on how to query the metrics. According to the go-metrics' godoc, EnableHostname is only applicable to gauge metrics:

“EnableHostname   	bool // Enable prefixing gauge values with hostname”

Consul initializes the metrics.config with the agent configuration value “telemetry.disable_hostname”.

consul/lib/telemetry.go

Lines 327 to 329 in 720dda7

	func configureSinks(cfg TelemetryConfig, memSink metrics.MetricSink) (metrics.FanoutSink, error) {
	metricsConf := metrics.DefaultConfig(cfg.MetricsPrefix)
	metricsConf.EnableHostname = !cfg.DisableHostname

Therefore, if disable_hostname is false, hostname will be added to gauge metric names (the default behavior); otherwise, no hostname will be prepended to the metrics name of gauge type. That said, disable_hostname has no impact on other types of metrics.

This PR clarifies this behavior in consul's doc.

Links

fix #17320

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

[wangxinyi7]

LGTM 👍

[jjti]

Suggested change

	This controls whether or not to prepend runtime telemetry of gauge type with the
	machine's hostname, defaults to false (prepend hostname).
	If enabled this disables prepending the name of gauge type metrics with the
	machine's hostname. By default, hostname is prepended.

[jjti]

possible rephrasing. I still find it a bit tricky to read as is. The section below is very helpful, though

[trujillo-adam]

I added a suggestion that I think will make the setting clearer.

[huikang]

@trujillo-adam , any idea what may cause the failed CI job?

https://github.com/hashicorp/consul/actions/runs/5182923956/jobs/9340224933?pr=17521