ddtrace/tracer: add Span.Tag function

[darccio]

What does this PR do?

Adds tracer.Span.Tag(key string) as part of its API to allow retrieving previously set tags.

Motivation

Simplify the error handling in cases like #2432 where our contribs may override already set errors.

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!

[rarguelloF]

LGTM!

[darccio]

Reverting this PR to fix #2432 as it would collide with future developments. We'll reopen #2432 and find an alternative.