feat: Support float64 gauges

[rzimmerman]

It can be useful to represent a Unix timestamp in a gauge. Prometheus metrics are float64s, so this works well when directly using the Prometheus libraries. However, because go-metrics Guage's are stored as float32s, there isn't enough precision to store a timestamp (https://go.dev/play/p/RvJ11Y8R7Cz) and times get truncated to about 2 minute intervals.

There's one instance of this in Vault: https://github.com/hashicorp/vault/blob/2dd4528ed8a8be83e0c1e0b48b1f23dab701181f/builtin/logical/pki/path_tidy.go#L1209
Which may be a bug.