Phosphor: Mark the extent of available history

Due to the way phosphor assigns blocks of the ring-buffer to different threads, we do not have the same time history for each thread.

When viewing phosphor trace spans in Chrome, it isn't always clear if there is not an event at a particular time on a thread due to it not occurring, or due to us having no events spanning that time period.

We should look at how we could improve this - perhaps simply emitting a span for the time range of each recorded block?

See also: MB-53379.