Simplify DynamicTransformer API

[felixbarny]

What does this PR do?

Instead of DynamicTransformer.Accessor.get().ensureInstrumented,users can just call DynamicTransformer.ensureInstrumented

Makes the API similar in style to the recent changes in WeakConcurrent.

Checklist

• This is something else
  • I have updated CHANGELOG.asciidoc

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2021-10-05T10:13:42.506+0000

• Duration: 57 min 6 sec

• Commit: 0df56e4

Test stats 🧪

Test	Results
Failed	0
Passed	2417
Skipped	19
Total	2436

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run benchmark tests : Run the benchmark test.

• run compatibility tests : Run the JDK Compatibility test.

• run integration tests : Run the APM-ITs.

[eyalkoren]

LGTM, only a minor naming suggestion

[eyalkoren]

[minor] I don't think DynamicTransformerSupplier is a good name for this interface. I would call it DynamicTransformer and find a better name for the declaring class, even StaticDynamicTransformer, but preferably something better 🙂

[felixbarny]

I agree that the name of the interface isn't ideal. But I wanted to optimize for the actual user facing class which is DynamicTransformer. While DynamicTransformerSupplier needs to be public, users never really get to see it. Also, it's in line with WeakConcurrentSupplier.

[eyalkoren]

It's just that DynamicTransformer doesn't supply anything (as opposed to the weak map supplier).
What about RuntimeInstrumentor for the interface?

And an additional suggestion - I just noticed that DynamicTransformerImpl seems redundant, why not make ElasticApmAgent the service provider implementing the interface (whatever it's called 🙂 ). ignore this one, it's static in ElasticApmAgent

Either way, feel free to ignore both suggestions and merge if you disagree. It's not that important.