internal/telemetry: do not send empty container and entity headers

[ahmed-mez]

What does this PR do?

Do not send container and entity ID headers if empty. It's already the case for profiling, dsm and tracing transport but the telemetry client was an anomaly.

Ideally, we should have a shared transport code that takes care of this.

Motivation

Fixes DataDog/system-tests#2021

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.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-01-16 10:43:06

Comparing candidate commit 0c5ea05 in PR branch ahmed/fix-telmemetry-headers with baseline commit 7d7770f in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.

[darccio]

LGTM

[ahmed-mez]

/merge

[dd-devflow]

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[ahmed-mez]

/merge

[dd-devflow]

❌ MergeQueue

You are not allowed to use the merge queue towards main.

If you need support, contact us on slack #ci-interfaces with those details!