lineMetrics is required for lineTrimOffset, but it's not documented

[mfazekas]

Environment

• Xcode version: 14.2
• iOS version: 16.2
• Devices affected: Simulator (iPhone 14 Pro Max)
• Maps SDK Version: 10.11.1, and #main( #3257f24 )

Observed behavior and steps to reproduce

lineTrimOffset doesn't work without lineMetrics. IMO it makes sense, but it's not documented.

In LineGradientExample.swift comment lineMetrics = true:

- geoJSONSource.lineMetrics = true // MUST be `true` in order to use `lineGradient` expression
+ // geoJSONSource.lineMetrics = true // MUST be `true` in order to use `lineGradient` expression

Also comment out the lineGradient part:

- lineLayer.lineGradient = .expression(
+ /* lineLayer.lineGradient = .expression( 

If you press Increase time offset the line will disappear and I see the following in the logs:

2023-03-11 19:32:04.835663+0100 Examples[40954:4605738] Failed to create render pipeline: Error Domain=CompilerError Code=2 "Vertex attribute a_packed(2) is missing from the vertex descriptor" UserInfo={NSLocalizedDescription=Vertex attribute a_packed(2) is missing from the vertex descriptor}
2023-03-11 19:32:04.836487+0100 Examples[40954:4605738] Can't create pipeline for `line` shader

Expected behavior

a.) document this at lineTrimOffset b.) throw a nicer error if possible

Notes / preliminary analysis

Doc for lineMetrics:

/// Whether to calculate line distance metrics. This is required for line layers that specify `line-gradient` values.

Doc for lineTrimOffset: /// The line part between [trim-start, trim-end] will be marked as transparent to make a route vanishing effect. The line trim-off offset is based on the whole line range [0.0, 1.0].

Additional links and references

#2676