Prevent warnings variable @metadata not initialised

[etiennechabert]

Issue #, if available:
#84

Description of changes:
Instead of directly using @metadata, use the metadata method in charge of doing an initialization when necessary

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[willarmiros]

LGTM! Thanks for contributing.

[willarmiros]

Actually it seems like this change caused tests to fail: https://github.com/aws/aws-xray-sdk-ruby/actions/runs/4292482419/jobs/7484285744#step:4:51

Could you try reproducing the test failure locally and seeing how we could fix this?

[etiennechabert]

@willarmiros should have paid more attention to what metadata method is doing...
not a regular memorization pattern there, actually two actions are being made:

• Initialize @metadata when necessary
• Initialize a sub_meta for a given namespace inside @metadata
• In the end returns a SubMeta object instead of a Metadata object, just a bit weird given the method name if I may.

Anyway, just adding a defined? to the condition is preventing the warning, without changing the behavior, working fine on my side when tested locally.

@willarmiros can you give me a new run on the tests 🙏 ?

[willarmiros]

Thanks for the change!