Probability sampling: Encode Span's head-adjusted count (open-telemetry#170)

* draft from OTEP 148

* renumber

* typo in header

* typos

* formatting

* clean TOC

* TOC edit

* Clarify the counting algorithm

* typos

* grammar

* grammar

* two paragraphs

* Summarize from the prototype

* Remove exported count from proposed spec language

* statement about not dropping sampler attributes

* from Thursday's SIG, limit proposal to head sampling probability

* log_head_adjusted_count

* Use 6 bits

* update the proto text

* add detail on SamplerResult

* remove metrics examples, add to span-to-metrics examples

* whitespace

* lint

Author: jmacd