core: more metrics (#5629)

## Summary
Add some more metrics:

- Authenticate token verification
- Authorization log duration
- Authorization evaluator and header evaluator
- IDP token session creator

HTTP and gRPC endpoints are already instrumented via middleware, which
covers authenticate, proxy and databroker endpoints. Postgres is also
already instrumented using `otelpgx`.

## Related issues
-
[ENG-2407](https://linear.app/pomerium/issue/ENG-2407/add-additional-metrics-and-tracing-spans-to-pomerium)


## Checklist

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

internal/telemetry/metrics/otel.go

@@ -31,3 +31,12 @@ func Int64Counter(name string, options ...metric.Int64CounterOption) metric.Int6
 	}
 	return c
 }
+
+// Int64Histogram returns an int64 histogram.
+func Int64Histogram(name string, options ...metric.Int64HistogramOption) metric.Int64Histogram {
+	c, err := Meter.Int64Histogram(name, options...)
+	if err != nil {
+		panic(err)
+	}
+	return c
+}