How to distinguish externally and locally originated errors for upstream clusters?

*Title*: How to distinguish externally and locally originated errors for upstream clusters?
*Description*:
> In order to setup proper alerting for Api Gateway based on Envoy I'm trying to find the opportunity to get metric that shows only the errors originated by Envoy.
The idea was to calculate the difference between `downstream_rq_xx` and `upstream_rq_<*xx>` but as I understand the `upstream_rq_<*xx>` includes also, for example, timeouts originated by Envoy.

Is there any stats for upstream cluster's externally originated errors?
If no, could you please suggest any workaround to get such statistics?

[optional *Relevant Links*:]
>I have found out that externally and locally originated errors are calculated separately during outlier detection:
https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/outlier
But I have found any related statistics for upstream errors.