[APM Tracing: Go] Update DD_TAGS documentation

[mtoffl01]

What does this PR do? What is the motivation?

Updates documentation on DD_TAGS. Default value of [] was incorrect because env var values are in string format. By default, no global tags are collected. Also updated the description to specify that key-value pairs must be string-convertible types.
Motivation: Customer request

Merge instructions

Merge readiness:

• Ready for merge

Merge queue is enabled in this repo. To have it automatically merged after it receives the required reviews, create the PR (from a branch that follows the <yourname>/description naming convention) and then add the following PR comment:

/merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/mikayla-toffler/patch-DD_Tags/tracing/trace_collection/library_config/go

[michaelcretzman]

@mtoffl01 Approved.
You can merge your PR by adding a comment containing just the command /merge.
The PR will go into the merge queue automatically once it's approved.
FYI - if the PR isn't approved within 2 hours after you add the /merge command you need to add the /merge command again.

[dd-devflow]

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---

2025-01-16 21:55:25 UTC ❌ MergeQueue

This PR is rejected because it was updated

[mtoffl01]

/merge

[mtoffl01]

Hey @michaelcretzman , I'm not sure what happened here? How can we merge this?
Thank you!

[michaelcretzman]

@mtoffl01 do you still need May's review? If not, she or I can merge it

[mtoffl01]

No, yours is sufficient! Thank you @michaelcretzman !