Document app.config limitation

[pellared]

Why

Fixes #1429

After discussion with @andrewlock and @Kielek we decided that it is safer and probably good enough NOT to support all configuration via env vars. The main problem are the configuration read by the .NET Profiler which does not know anything about app.config. We could try "refreshing" the native configuration after the managed layer has loaded the app.config. However, that would probably require PInvoke and additional synchronization. Still this might result in race conditions.

Moreover, that env vars are per-process, but web.config can be per-appdomain; e.g.

• App1 and App2 are web apps running in IIS
• App1 sets OTEL_SOME_SETTING=1 in web.config
• App2 sets OTEL_SOME_SETTING=0 in web.config
• App1 sets the env var OTEL_SOME_SETTING=1 and Pinvokes to notify the profiler
• App2 sets the env var OTEL_SOME_SETTING=0 and Pinvokes to notify the profiler
  Profiler doesn't know what to think
  Essentially you'd have to check the appdomain for each callback, and some parts of the profiler are agnostic to that
  So things will get complicated quickly.

At last this functionality would hopefully be not needed for the "happy path". The usual scenario would be use the app config to set the service name and other resource attributes, configure the exporter(s).

What

Document app.config and web.config limitation.

Tests

Manually by modyfing integration tests and test applications

Checklist

• CHANGELOG.md is updated.
• Documentation is updated.
• New features are covered by tests.

[theletterf]

Might require some extra work, but I'm thinking now of re-doing the settings tables so that they have the following columns:

• Setting (env var)
• App.config / Web.config supported
• Description
• Default

[pellared]

Can we keep as it is for this PR (it makes also reviewing easier)? We can change the formatting/structure it in a separate PR.

[theletterf]

@pellared Perhaps we can be more prescriptive as to when a method should be used instead of another? If there's choice, that is. Of course one can use env vars for everything, but when should they use app.config instead?

[pellared]

@pellared Perhaps we can be more prescriptive as to when a method should be used instead of another? If there's choice, that is. Of course one can use env vars for everything, but when should they use app.config instead?

#2071

[zacharycmontoya]

LGTM with very small comments