[V2 experimentation] ddtrace/tracer: have tracer.Start return an error

[JianyiGao]

What does this PR do?

Added err type in tacer.Start() return value
Proposal: #540

Motivation

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• 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.

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: 2023-10-27 13:35:44

Comparing candidate commit 7bfb9c2 in PR branch jennie.gao/tracer-start-error with baseline commit e2429fc in branch jennie.gao/v2-experimentation-main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 41 metrics, 0 unstable metrics.

[dianashevchenko]

Looks good, I would advise to check all the places with returned errors to make sure they look presentable, that is returning that error actually brings value to the customer, as opposed to confusing them

[JianyiGao]

Moving all V2 prs to point against v2-dev