feat: Add Bugsnag.getMetadata()

[robinmacharg]

Goal

Provide metadata retrieval from the global Bugsnag class.

Design

A class method was added to the Bugsnag class, exposed appropriately to Swift. Equivalent lower-level methods were renamed to align them with the implemented method. The functionality was changed from "create if not available" to "return nil if not available".

Changeset

Bugsnag and BugsnagMetadata, along with appropriate unit test files.

Tests

Unit tests were provided in Objective C (and Swift to confirm name exposure).

Review

Outstanding Questions

• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

• The correct target branch has been selected (master for fixes, next for
  features)
• Consistency across platforms for structures or concepts added or modified
• Consistencyxbetween the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[fractalwrench]

I had a few queries about whether we need to implement additional methods for manipulating metadata that are listed in the spec. What has been implemented looks good - I left a few comments inline, some of which are more queries about how Objective-C works.

One thing to note is that the objective-c-ios example app still seems to be using event.metaData and therefore fails to compile, so it'd be worth checking that doesn't affect any of the other example apps.

[robinmacharg]

Thanks for the detailed feedback - dealt with individually inline.

One thing to note is that the objective-c-ios example app still seems to be using event.metaData and therefore fails to compile

This is an easy one to miss (it's not the first time I have). There's a larger piece of work needing to be done to unify the three Apple platforms in a single project which will simplify the burden going forwards. The original reason for the separation (Carthage issues) has been resolved.

For now I'll update the PR template, adding an explicit check for the example app (as well as fixing the example apps).