[configuration] Allow ${env:ENV} notation for environment variable substitution

[mx-psi]

Fixes #3961

Changes

Allows for the usage of the ${env:ENV} syntax in SDK file configuration. This syntax is used in the OpenTelemetry Collector, see https://opentelemetry.io/docs/collector/configuration/#environment-variables for more information.

This does not mean that other providers need to be supported; if we want to allow this we would need to reserve that syntax by rejecting ${<provider>:URI} as invalid, since right now it would be left as-is.

• Related issues Support for ${env:ENV} syntax #3961, part of Alignment with OpenTelemetry Collector configuration #3963

[mx-psi]

cc @open-telemetry/configuration-maintainers

[jack-berg]

I'm in favor of this for an improved compatibility story with the collector env vars substitution syntax. I'm not in favor of expanding the set of prefixes to account for other sources like those supported in the collector since doing so adds complexity to implementations which needs to be justified with user demand / use cases.

[tigrannajaryan]

Do we want ${env:ENV} syntax in addition to ${ENV}? Did we consider it to be instead so that there is only one syntax?

[mx-psi]

Do we want ${env:ENV} syntax in addition to ${ENV}? Did we consider it to be instead so that there is only one syntax?

I would be in favor of only supporting ${env:ENV}, as I think most/all of @open-telemetry/collector-triagers would be. I am not sure what the opinion of the Configuration WG is, this was explicitly discussed in the past IIRC.

[dmitryax]

Right, the collector SIG decided in favor of ${env:ENV} to stay consistent with other config sources

[jack-berg]

Do we want ${env:ENV} syntax in addition to ${ENV}? Did we consider it to be instead so that there is only one syntax?

There was pushback on the env: prefix because most users will be more familiar with the bash syntax. More details about the conversation in this comment: #3961 (comment)

This was discussed in the spec SIG in addition to the file config SIG.

[codeboten]

I agree that it makes sense to support env: prefix. Since not having the prefix was the preference of the config working group, i'm ok with moving forward supporting both (as we do today in the collector)