Introducing ContextSnapshotFactory

[chemicL]

This change is mainly allowing clearing ThreadLocals upon scope enter.

There are however more changes to the API:

• Added a ContextSnapshotFactory concept, that allows configuring with a ContextRegistry, the Predicate to use when capturing, and a boolean flag whether missing mappings in a context object should clear existing ThreadLocals or not touch them.
• Deprecated existing static methods in ContextSnapshot and made them use a global instance of ContextSnapshotFactory which does not clear missing values.
• Forked the DefaultContextSnapshot test to cover the deprecated methods.
• Used a ContextSnapshotFactory.Builder in tests to depict the intended usage.

[sonatype-lift]

🛠 Lift Auto-fix

Some of the Lift findings in this PR can be automatically fixed. You can download and apply these changes in your local project directory of your branch to review the suggestions before committing.¹

# Download the patch
curl https://lift.sonatype.com/api/patch/github.com/micrometer-metrics/context-propagation/105.diff -o lift-autofixes.diff

# Apply the patch with git
git apply lift-autofixes.diff

# Review the changes
git diff

Want it all in a single command? Open a terminal in your project's directory and copy and paste the following command:

curl https://lift.sonatype.com/api/patch/github.com/micrometer-metrics/context-propagation/105.diff | git apply

Once you're satisfied, commit and push your changes in your project.

Footnotes

1. You can preview the patch by opening the patch URL in the browser. ↩

[rstoyanchev]

Maybe we can take the opportunity to streamline the API a little.

For a start, I think we can collect a keyPredicate in the builder, in effect expecting the factory to be pre-configured with it rather than having them passed in. Capturing tends to be a lot more fixed and inclusive, so I expect this should be just fine.

Then for the methods:

ContextSnapshot capture(Object... contexts);
ContextSnapshot captureFromContext(Object... contexts); 
<C> ContextSnapshot.Scope setThreadLocalsFrom(Object sourceContext, String... keys);

For setThreadLocalsFrom if the keys is empty, set all.

[chemicL]

@rstoyanchev thanks for the suggestion. I applied the change to the draft, let me know your thoughts. Keep in mind the DefaultContextSnapshot and DefaultContextSnapshotFactory implementations need to provide backwards compatibility to allow for the deprecated methods to behave as before.

[rstoyanchev]

One more thing, I see the setThreadLocalsFrom static methods are moved to DefaultContextSnapshotFactory but not the capture static methods. I think it would make sense to have that logic in the factory.

[rstoyanchev]

Thanks for applying the changes.

I think there may be some further opportunity to consolidate logic across the setThreadLocals methods, but to minimize moving parts, we can keep it at that for now, and try another pass after the PR is in.

Some minor code ordering comments below.

[rstoyanchev]

Overall looks good. Some minor suggestions below.