expfmt: OpenMetrics 1.0 output with a unit that is not a name suffix fails the whole scrape in Prometheus

[Rohilalala]

Since #877, MetricFamilyToOpenMetrics no longer adds the unit to the metric name, but it still writes the # UNIT line. OpenMetrics 1.0 requires the unit to be a suffix of the name, and Prometheus's OpenMetrics 1.0 parser rejects the exposition, so every metric on the target is lost (the same failure as prometheus/client_golang#1990).

Reproduction with client_golang v1.24.1:

reg := prometheus.NewRegistry()
reg.MustRegister(prometheus.NewCounter(prometheus.CounterOpts{
	Name: "request_duration_total", Help: "Time spent.", Unit: "seconds",
}))
// promhttp.HandlerFor(reg, promhttp.HandlerOpts{EnableOpenMetrics: true})

# HELP request_duration Time spent.
# TYPE request_duration counter
# UNIT request_duration seconds
request_duration_total 1.5
# EOF

Prometheus: unit "seconds" not a suffix of metric "request_duration".

Options I can see:

1. The OpenMetrics 1.0 encoder omits # UNIT when the unit is not a suffix, the same way it already writes a counter without _total as unknown "to avoid invalid OpenMetrics output". Names are never changed. (Test case 16 would change.)
2. Prometheus's OpenMetrics 1.0 parser stops rejecting it, following OpenMetrics 2.0, where the suffix is only a SHOULD.

Happy to send a PR for whichever is preferred.