fix(core): serialize bigint in metadata to string

[dreamorosi]

Issue #, if available: closes #616

Description of changes:

This PR introduces a new method called serialize to the prototype of the Segment and Subsegment objects. This method wraps the JSON.stringify() call that was previously made by the toString() and format() methods of these two classes.

When calling the JSON.stringify() function, the method also passes a replacer function. This replacer for now includes an if statement that casts BigInt objects to string. This avoids runtimes errors described in the linked issue.

By extracting the method, customers can easily override it and extend the replacer to suit their needs.

In addition to the method, the PR includes also changes to the type definition and the addition of a test.

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

[misterjoshua]

Looks pretty good. I have a few suggestions.

[misterjoshua]

I'd like to see a similar test for Segment

[dreamorosi]

The existing test harness for Segment stops at mocking the Segment.flush() method and asserting that it's been called. Given that segment is serialized within the flush method, the serialization is never done.

Adding this test would mean rearchitecting the tests and requires knowledge of the codebase that I don't have.

If the maintainers want to add this, I'd suggest them to either: 1/ address this in a future PR, 2/ contribute to this PR and provide the test fixture.

[misterjoshua]

LGTM!

[carolabadeer]

Thanks for making this change! Left a few small comments, but otherwise looks good

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a9f999) 83.41% compared to head (4a2e140) 83.49%.
Report is 1 commits behind head on master.

❗ Current head 4a2e140 differs from pull request most recent head b3e3567. Consider uploading reports for the commit b3e3567 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #619      +/-   ##
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+ Coverage   83.41%   83.49%   +0.07%     
==========================================
  Files          37       37              
  Lines        1797     1805       +8     
==========================================
+ Hits         1499     1507       +8     
  Misses        298      298              

see 3 files with indirect coverage changes

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[dreamorosi]

Hi @carolabadeer thanks for the review and the comments, I have committed both your suggestions!