Improve markdown table generation for metrics

[arminru]

(split from open-telemetry/build-tools#79)

See https://github.com/open-telemetry/build-tools/pull/79/files#r996803040 for the previous discussion and proposals.

Two possible options would be:

1. A table of metric names with a (then shared) table of attribute names that apply.
2. A table of the metric where "empty" rows are created while attributes are expanded.

Both of them are currently manually applied in the hand-crafted metrics tables in the spec repo.

cc @jamesmoessis @Oberon00 @joaopgrassi @jsuereth