Support float64 gauge metrics

[rzimmerman]

Is your feature request related to a problem? Please describe.
It can be useful to report Unix timestamps as gauge metrics. For Prometheus, the recommended unit is Unix timestamps (in seconds) https://prometheus.io/docs/practices/naming/. Vault uses the go-metrics library which stores metrics as float32s, later upcast for Prometheus. For Unix times, this isn't enough precision (https://go.dev/play/p/RvJ11Y8R7Cz) and truncates the value to about ~2 minute precision.

Vault has at least one instance of this, probably unnoticed:

vault/builtin/logical/pki/path_tidy.go

Line 1209 in 2dd4528

metrics.SetGauge([]string{"secrets", "pki", "tidy", "start_time_epoch"}, float32(b.tidyStatus.timeStarted.Unix()))

Describe the solution you'd like
Either update go-metrics to use float64 values, or add a Gauge64 (something like this fork https://github.com/rcrowley/go-metrics/blob/cf1acfcdf4751e0554ffa765d03e479ec491cad6/gauge_float64.go)

Describe alternatives you've considered
Storing timestamps with a different epoch would work, but be difficult to integrate with tools like Grafana. It also just kicks the can down the road.
Recording the metric as a "time since event"/"age" would also work, but is brittle in cases where metrics have gaps.

Explain any additional use-cases
Specifically for Vault Agent, it is useful to record things like last authentication time and other timestamps for alerting.

go-metrics issue opened here: hashicorp/go-metrics#150

[biazmoreira]

Blocked by #21130