docs: Clarify UPSTREAM_REMOTE_ADDRESS and UPSTREAM_HOST access log variables are the same

[GUI]

These access log variables use identical code so they should always be equivalent. I was initially wondering how these variables differed, but after digging in, I found they have identical source code, so I thought this might be worth pointing out in the docs that these reflect the same value.

I suppose one of these could be deprecated if you all wanted to eliminate the duplication, but that would obviously be a bigger change. In the meantime, I thought clarifying the fact that they are the same might help clear up any confusion, like my own. It seems like UPSTREAM_REMOTE_ADDRESS was added more recently (#19613) and is part of a more consistent set of variables, but UPSTREAM_HOST has been around much longer.

Current identical implementation between these two variables on main:

envoy/source/common/formatter/stream_info_formatter.cc

Lines 876 to 887 in 6092cdc

	{"UPSTREAM_HOST",
	{CommandSyntaxChecker::COMMAND_ONLY,
	[](const std::string&, absl::optional<size_t>) {
	return StreamInfoAddressFormatterProvider::withPort(
	[](const StreamInfo::StreamInfo& stream_info)
	-> std::shared_ptr<const Envoy::Network::Address::Instance> {
	if (stream_info.upstreamInfo() && stream_info.upstreamInfo()->upstreamHost()) {
	return stream_info.upstreamInfo()->upstreamHost()->address();
	}
	return nullptr;
	});
	}}},

envoy/source/common/formatter/stream_info_formatter.cc

Lines 941 to 952 in 6092cdc

	{"UPSTREAM_REMOTE_ADDRESS",
	{CommandSyntaxChecker::COMMAND_ONLY,
	[](const std::string&, absl::optional<size_t>) {
	return StreamInfoAddressFormatterProvider::withPort(
	[](const StreamInfo::StreamInfo& stream_info)
	-> std::shared_ptr<const Envoy::Network::Address::Instance> {
	if (stream_info.upstreamInfo() && stream_info.upstreamInfo()->upstreamHost()) {
	return stream_info.upstreamInfo()->upstreamHost()->address();
	}
	return nullptr;
	});
	}}},

Also the same implementation on the v1.27.0 tag:

https://github.com/envoyproxy/envoy/blob/v1.27.0/source/common/formatter/substitution_formatter.cc#L1128-L1140
https://github.com/envoyproxy/envoy/blob/v1.27.0/source/common/formatter/substitution_formatter.cc#L1205-L1217

Commit Message: docs: Clarify UPSTREAM_REMOTE_ADDRESS and UPSTREAM_HOST are the same
Additional Description: N/A
Risk Level: Low (docs only)
Testing: N/A
Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

[repokitteh-read-only]

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🐱

Caused by: #29800 was opened by GUI.

see: more, trace.

[repokitteh-read-only]

Hi @GUI, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #29800 was opened by GUI.

see: more, trace.

[ggreenway]

Yep, they are identical. Thanks for clarifying the docs!

Please fix DCO in your PR, as described here: https://github.com/envoyproxy/envoy/blob/main/CONTRIBUTING.md#fixing-dco

/wait

[GUI]

@ggreenway: Ah, sorry I missed the DCO stuff. That should be fixed in the latest commit. Thanks for pointing that out!

[ggreenway]

I suppose one of these could be deprecated if you all wanted to eliminate the duplication

I think it's probably not worthwhile. It would require getting people using the deprecated one to change their config, and it's not very difficult to keep supporting both forever.