ddtrace/tracer: update the expected telemetry tags for OTel env-var mapping

[mcculls]

What does this PR do?

The expected telemetry tags when reporting OTel env-var mapping issues now use snake-case:

 config.datadog       -> config_datadog
 config.opentelemetry -> config_opentelemetry

The expected tag values should also be lower-case.

Motivation

The RFC has been revised since #2715

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.
• 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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-07-25 12:38:07

Comparing candidate commit f09a5fb in PR branch mcculls/otel-env-var-metric-tag-update with baseline commit 05a8b5a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 46 metrics, 1 unstable metrics.

[darccio]

@mcculls Can we safely assume that the telemetry backend will handle both formats, right?

[mcculls]

@mcculls Can we safely assume that the telemetry backend will handle both formats, right?

The metric definitions were only recently added to telemetry intake allow list in https://github.com/DataDog/dd-go/pull/142947 (using the latest format from the RFC - the reason for the change was that the dotted names are not valid tag names for telemetry)