[PLAT-3941] Reduce metadata churn

[nickdowell]

Goal

The metadata changed blocks were being called in cases where the data had not actually changed.

This is inefficient, particularly now that metadata is written to disk when it changes.

Changeset

The BugsnagMetadata class has been updated to only notify observers if the data has changed. This required adjustments to a few unit test cases that were relying on the old broken behaviour.

BugsnagClient has been updated to

• stop adding itself as an observer twice
• add app and device info before registering as an observer

Also uncovered a bug whereby the user information would not be written to disk - this was due to the use of the _user ivar.

Testing

Added a new unit test case to verify that observer is not called if data has not changed.

Manually tested using a sample app...

Before

2020-12-04 09:18:08.213933+0000 objective-c-osx[28729:1185082] metadata.json = {
}
2020-12-04 09:18:08.214373+0000 objective-c-osx[28729:1185082] config.json = {
    appVersion = "1.0";
    bundleVersion = 1;
    releaseStage = development;
}
2020-12-04 09:18:08.214765+0000 objective-c-osx[28729:1185082] state.json = {
}
2020-12-04 09:18:08.215201+0000 objective-c-osx[28729:1185082] metadata.json = {
}
2020-12-04 09:18:08.236219+0000 objective-c-osx[28729:1185082] metadata.json = {
    app =     {
        name = "objective-c-osx";
    };
}
2020-12-04 09:18:08.236677+0000 objective-c-osx[28729:1185082] metadata.json = {
    app =     {
        name = "objective-c-osx";
    };
}
2020-12-04 09:18:08.237120+0000 objective-c-osx[28729:1185082] metadata.json = {
    app =     {
        name = "objective-c-osx";
    };
    device =     {
        simulator = 0;
        timezone = GMT;
        wordSize = 64;
    };
}
2020-12-04 09:18:08.237552+0000 objective-c-osx[28729:1185082] metadata.json = {
    app =     {
        name = "objective-c-osx";
    };
    device =     {
        simulator = 0;
        timezone = GMT;
        wordSize = 64;
    };
}

After

2020-12-04 09:19:08.719311+0000 objective-c-osx[28960:1187203] metadata.json = {
    app =     {
        name = "objective-c-osx";
    };
    device =     {
        simulator = 0;
        timezone = GMT;
        wordSize = 64;
    };
}
2020-12-04 09:19:08.719858+0000 objective-c-osx[28960:1187203] config.json = {
    appVersion = "1.0";
    bundleVersion = 1;
    releaseStage = development;
}
2020-12-04 09:19:08.720317+0000 objective-c-osx[28960:1187203] state.json = {
}
2020-12-04 09:19:08.721029+0000 objective-c-osx[28960:1187203] state.json = {
    user =     {
        id = 93f3d632b7fcb0365071780cf1f3e6229c1be887;
    };
}