Support the OTEL_TRACES_EXPORTER environment variable

[MrAlias]

Add support for the OTEL_TRACES_EXPORTER environment variable as defined in the specification.

This will require designing and ratifying a plan to support loading all the exporters we need without having the SDK import them all. As was mentioned here we could look at the https://pkg.go.dev/plugin package. Or, as was suggested in a SIG meeting, look into maybe some registration method similar to the database/sql package.

It might also be the case that we do not want to ultimately support this. Part of this issue is determining the feasibility and appropriateness of this configuration option.

Part of #1698

[MovieStoreGuy]

@MrAlias , moving to the https://pkg.go.dev/plugin requires that applications ship with cgo enabled since it depends on the c method dlopen. This could be problematic for services that ship a static binary into their production environment. I believe this to be the common case for most golang based applications.

Is it permissible to have environment variable defined, but only consumed if the application developer adds in the exporter explicitly?

for example:

import (
     otel/sdk
     otel/exporter
     _ otel/exporter/zipkin // registering global init method to allow for zipkin exporter
)

// Carrying on, start the SDK and have a valid, configured zipkin exporter

import (
    otel/sdk
    otel/exporter
    _ otel/exporter/otlp
)

// zipkin var is present but no zipkin exporter configured

[pellared]

look into maybe some registration method similar to the database/sql package

I see it as the best approach as

1. it does not force any dependencies
2. it gives the possibility to select the propagators that are supported by the application
3. it allows also building a convenience module/package that will contain the propagators that should be supported by the spec (tracecontext and baggage)
4. I find it good when we can reuse a pattern used in the standard library

https://pkg.go.dev/plugin is rather a no-go as it does even not work on all platforms. I find it also not suitable for this use case.

FYI this is something I plan to do in .NET SDK: open-telemetry/opentelemetry-dotnet#1453 (comment)

[MovieStoreGuy]

I know myself and few others have toyed with idea of connivence libs to effectively meet the same goal.

An example to help get me get started quicker is https://github.com/MovieStoreGuy/otel-go-starter/tree/main.

it does not force any dependencies

Just to double check what you mean, are you meaning that there is no tightly coupled code inside the sdk?
Or that we should only include the required exporter code (say .../pkg/zipkin) when explicitly asked to?

[pellared]

I mean "we should only include the required exporter code (say .../pkg/zipkin) when explicitly asked to".

[MrAlias]

Should be resolved by open-telemetry/opentelemetry-go-contrib#2753

[pellared]

I think that the env var should be supported by the SDK (sdk/trace package).

Reference: open-telemetry/opentelemetry-specification#3721 (comment)

[pellared]

On hold. See open-telemetry/opentelemetry-specification#3721 (comment)

[pellared]

Closing per open-telemetry/opentelemetry-specification#3730