metrics: don't add units and scope tags to prometheus metrics (#5749)

## Summary

With metrics now going via OTEL -> Prometheus exporter, there were
couple issues:

- some newer metrics had non-standard units that ended up in metric
names like `pomerium_storage_global_cache_hits__hit__total`
- we had three additional tags (`otel_scope_name`,
`otel_scope_schema_url`, `otel_scope_version`) added to each metric,
which do not really carry much meaningful information for us and expand
metric cardinality.

This PR reverts those two changes and updates the prometheus exporter.

## Related issues

Related: https://github.com/open-telemetry/opentelemetry-go/issues/7039

## User Explanation

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change doesn't create any user-facing changes, you can leave
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## Checklist

- [x] reference any related issues
- [ ] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review

internal/telemetry/metrics/otel.go

@@ -11,7 +11,11 @@ import (
 var Meter metric.Meter
 
 func init() {
-	e, err := prometheus.New(prometheus.WithNamespace("pomerium"))
+	e, err := prometheus.New(
+		prometheus.WithNamespace("pomerium"),
+		prometheus.WithoutUnits(),
+		prometheus.WithoutScopeInfo(),
+	)
 	if err != nil {
 		panic(err)
 	}