[iOS] Add turnstile events for Snapshotter

[lloydsheng]

Snapshotter (i.e. https://www.mapbox.com/android-docs/maps/overview/snapshotter) is part of the Maps SDK and should be sending turnstile events before usage on both iOS and Android (Snapshotter requests need the same access token we use for regular tile requests).
We should add turnstile events for Snapshotter.

[julianrex]

I think this should be in -startWithQueue:completionHandler: - an MGLMapSnapshotter may be allocated and not used, or used multiple times.

Perhaps sending the turnstile event somewhere around

mapbox-gl-native/platform/darwin/src/MGLMapSnapshotter.mm

Lines 201 to 210 in 53351c2

	self.completion = completion;
	self.resultQueue = queue;
	self.cancelled = NO;
	__weak __typeof__(self) weakSelf = self;
	// mbgl::Scheduler::GetCurrent() scheduler means "run callback on current (ie UI/main) thread"
	// capture weakSelf to avoid retain cycle if callback is never called (ie snapshot cancelled)
	_snapshotCallback = std::make_unique<mbgl::Actor<mbgl::MapSnapshotter::Callback>>(

would make sense?

[julianrex]

It looks like Android sends it in the initializer too: #13475

Which should it be? /cc @tobrun

[lloydsheng]

@julianrex I think the behavior should consist with MGLMapview did.

mapbox-gl-native/platform/ios/src/MGLMapView.mm

Lines 629 to 632 in 8bc362e

	if ([UIApplication sharedApplication].applicationState != UIApplicationStateBackground) {
	[MGLMapboxEvents pushTurnstileEvent];
	[MGLMapboxEvents pushEvent:MMEEventTypeMapLoad withAttributes:@{}];
	}

The turnstile events send in the initializer of mapview.

[lloydsheng]

@julianrex @tobrun What's your opinion? Can I merge this PR?

[julianrex]

We can go ahead and merge this for consistency (with the existing landed Android PR) - though I'd like to fully understand why it shouldn't be in the start... method.

[springmeyer]

We can go ahead and merge this for consistency (with the existing landed Android PR) - though I'd like to fully understand why it shouldn't be in the start... method.

Is there a ticket tracking this question? We should get to a resolution. /cc @lloydsheng @julianrex