Add flow or async event around raster cache populate->eviction events

[flar]

We don't provide any statistics on how long cache entries remain in use. It would be nice if the creation and deletion of an individual cache entry added events to the timeline then we could visualize and potentially graph that time frame.

[chinmaygarde]

After staring at traces with @goderbauer, I think we preferred flow events because we could select a whole bunch of them and analyze/graph their durations. This would give us a good idea about how long each entry sticks around in the cache.

[github-actions]

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