Need to add clarification to metric thresholding to call out that it only supports `gauge` type metrics

[asachs01]

Description

Category

This is an issue with:

• [ ] errata (error in doc content)
• [ ] new doc (requires a completely new doc)
• [x] update (add missing or refresh existing content)
• [ ] improvement (improve docs content or format)

Affected pages

https://docs.sensu.io/sensu-go/latest/observability-pipeline/observe-schedule/metrics/#metric-threshold-evaluation

Corrections needed

Metric thresholding only supports gauge metrics and that should be very clear from the docs.