Update 0009-metric-handles to match current spec

[jmacd]

This OTEP reflected a relatively-complicated association between instruments, their recommended keys, and handles. Much of this text was no longer needed. In particular, a large "Trade-offs and mitigations" block of text was completely rewritten as 0049-metric-label-set.md.

This brings this OTEP into line with the current spec. I believe there are no significant discrepancies between this and the current spec. In a future OTEP I will propose the renaming of "Handle" to "Bound instrument". Note, however, that "Handle" is not a recommended type name, it is only a concept. Each kind of instrument has a distinct type of handle, but in discussing this feature, we will use "Bound instrument" in the future.

[lzchen]

Just saw the comment about LabelSets. Reviewing OTEPs in retrospective is difficult, so as long as the linking is evident it should be good.

[c24t]

For anyone else struggling to keep up with metrics work: open-telemetry/opentelemetry-specification#299 made significant changes to the metrics spec that weren't yet described by OTEP 0009. The specs PR was informed by go prototypes at open-telemetry/opentelemetry-go#100 and open-telemetry/opentelemetry-go#172.

The specs changes were merged, but the OTEP was still "proposed" (i.e. not yet "accepted"). In the meantime #49 was merged, and the process was changed (in #55) so that OTEPs wouldn't be merged until they were "accepted". This PR revises the OTEP and moves it to "accepted". Usually the next step would be to edit the spec to reflect the OTEP, but in this case that's already done.

[jmacd]

Thank you. I will merge this tomorrow.

[jmacd]

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