What does this PR do?

This PR adds a new package internal/env that wraps os.Getenv and os.LookupEnv calls, allowing only known environment variables to be used.

All environment variables/configuration keys are now listed in internal/env/supported_configurations.json and then generated as a Go map in internal/env/supported_configurations.gen.go after running the generate command.

This PR includes new CI steps/jobs:

• A golangci-lint rule (forbidigo) that forbids the use of os.Getenv and os.LookupEnv here
• A config-inversion step that runs after go test (or equivalent) steps to check for the introduction of new configuration keys here
• Config-inversion GitLab jobs that check existence against the configuration registry and mark configuration keys upon version release here
  • Example of keys not existing in the registry: failed job + second run with keys added successful job

Most interesting parts to review:

• internal/env package
  • Contains the wrapping and validation of os calls
  • Includes code that runs only during unit tests (limited by testing.Testing() being true only in go test scenarios) that adds environment variables to the JSON file
• instrumentation/env package
• configinverter script
• CONTRIBUTING.md documentation for environment variables
• DD-API-KEY handled via alias system part of config inversion.

Most of the changes involve migrating os calls and adding new tests to cover missing environment variables.

Motivation

This PR main goal is to control the use of environment variables in the tracer so that we have a way to list and document every available configuration.

There is a new configuration registry implemented on the feature parity dashboard that contains all the configuration keys for every tracer.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running golangci-lint run locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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