* Pass Go context down to Renderers
This is useful for cancellation or tracing.
* Add tracing spans to app
Also log things like number of nodes in Map, total number of reports.
instead of a Map, since it's not really a map as it just updates the
given node.
This is more efficient and also matches what we do in similar
situations elsewhere, e.g. in ContainerWithImageNameRenderer and
ProcessWithContainerNameRenderer.
* Helps reduce garbage (MakeMetric() now takes a slice and there's a shorter version MakeSingletonMetric())
* Fixes bug computing Max (Min) in samples since using MakeMetric()
was causing a default Max/Min of zero.
* Simplifies code a bit
