Update metrics temporality

[brettmc]

I think our default implementation might be based on LowMemory temporality

Yes, we use LowMemory by default (declarative and env based config are already using Cumulative by default).

not sure if the spec has changed since this was implemented

Our metrics implementation was written before LowMemory was added to the spec / the default temporality was changed from SHOULD to MUST ~11 months ago. Note that our interpretation of Delta and LowMemory is also out of sync with the spec (UpDownCounters should always use Cumulative temporality). Not sure whether this should/can be fixed in 1.x, but we should definitely fix it for 2.0.

Originally posted by @Nevay in #1589