Extendibility via plugins

[RassK]

Why

Distro providers should have more simpler ways to build up their own distro. Full fork is hard to manage and keep in sync.

What

Introduce plugin architecture for extendibility. Plugins could reside in /plugins folder where dlls can be dynamically loaded.

This feature is connected to: #118

[pellared]

We need to agree on what extension points are needed.

For Splunk, we need to be able to add additional (or modify existing) HTTP response headers for HTTP Inbound instrumentation.

Maybe we should start with this one?

[pellared]

Instead of

Plugins could reside in /plugins folder where dlls can be dynamically loaded.

I would prefer to add some env var like OTEL_DOTNET_INSTR_PLUGINS. Requiring to explicitly set it is more secure. It also gives the user to use different plugins for different application. It also makes it possible to harden the location where the plugin resides (e.g. aggressively restrict access to it as potential place where a malicious user could inject some sniffer).

[pellared]

For reference: open-telemetry/opentelemetry-java-instrumentation#566