Enable span suppression by type (SpanKey) by default

[mateuszrzeszutek]

#3691 introduced the more flexible, "by type" span suppression - it is still turned off by default (code), even though the spec allows nested CLIENT/PRODUCER span kinds (link).
Should we turn this behavior on by default? Recently there's been some progress on the instrumentation-api stability front, and we have to decide what we're going to do with this feature if we want to release a stable version. Personally, I believe that this should be on by default - since it very much makes sense to emit multiple spans of the same kind if they describe different aspects (conventions) of an operation.

[trask]

I support enabling it by default.

cc @lmolkova