xds: Add passive health check config for upstreams (aka envoy outlier detection)

[dnephin]

Closes #6832

Adds support for configuring two of the many outlier detection settings which provide passive health checking. These two configuration options are approximately the two options supported by HAProxy and NGinx.

The new configuration is being added to proxy.upstreams[].config. Envoy has a number of settings (enabled by split_external_local_origin_errors) that change the behaviour for local vs remote errors. I think if we were to configure these passive health check settings from the proxy config of the destination service (instead of the upstream config of the source service), it may be difficult to conceptualize "local".

I am opening this PR with a couple of open TODOs to get feedback on the naming and location of the config (UpstreamConfig).

TODO

• docs
• some coverage of the new config from the envoy integration suite

[crhino]

I think the naming seems reasonable to me. passive_health_check threw me off for a bit until I read through Envoy and Nginx docs and realized they both referred to these types of checks as passive.

max_failures also seems reasonable, especially since I don't see a compelling reason to enable split_external_local_origin_errors.

[mkeeler]

Looks good to me.

It should just work with all the various Proxy config merging but it would be good when going through the integration tests to ensure that setting these in various places trickles down properly.

• proxy-defaults/global config entry
• service-defaults/<service name>
• In the individual proxy configuration: Upstream.Config.

[dnephin]

I added some passive_health_check config to one of the integration tests. The max_failures field is working, but interval is not for some reason. I'll continue to debug. Pushed a commit to see if it is broken on all versions of envoy.

[dnephin]

Tests are passing! This is ready for a full review now.

[crhino]

Suggested change

	- `passive_health_check` - Settings for the load balancer, used to remove
	- `passive_health_check` - Settings for the proxy, used to remove

nit, but I would use the word proxy instead of load balancer so as not to introduce another noun into the mix

[crhino]

A couple of small comments, but nothing blocking or big, good work! Just worked when I tested it out. 👍

[crhino]