ddtrace/tracer: simplified SpanTagsResourceRule

[dianashevchenko]

What does this PR do?

Updates SpanTagsResourceRule according to the comment left on main feature branch.
Additionally, updates the MarshalJSON function with the new fields.

Motivation

Simplify UX.

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.

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: 2023-12-28 12:15:04

Comparing candidate commit 44b0590 in PR branch shevchenko/simplify-sampling-option with baseline commit 5d50d35 in branch shevchenko/tag-resource-sampling.

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