Deprecate metric/unit and use a string for units in the metric API/SDK

[MrAlias]

Part of #3775

The OTel specification states a unit needs to be a string that is not validated by the API or SDK, and the API design of the unit package is brittle and will not evolve well. This deprecates the unit package so it will be removed in the next release, and replace all use of the Unit type with a string throughout the metric signal.

[codecov]

Codecov Report

Merging #3776 (1a2cf87) into main (17e5d0f) will decrease coverage by 0.1%.
The diff coverage is 100.0%.

❗ Current head 1a2cf87 differs from pull request most recent head a934378. Consider uploading reports for the commit a934378 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3776     +/-   ##
=======================================
- Coverage   81.7%   81.7%   -0.1%     
=======================================
  Files        167     167             
  Lines      12462   12450     -12     
=======================================
- Hits       10184   10172     -12     
  Misses      2063    2063             
  Partials     215     215             

Impacted Files	Coverage Δ
exporters/otlp/otlpmetric/internal/otest/client.go	97.1% <ø> (ø)
exporters/prometheus/exporter.go	83.5% <ø> (ø)
sdk/metric/metricdata/data.go	57.1% <ø> (ø)
sdk/metric/pipeline.go	90.5% <ø> (-0.1%)	⬇️
bridge/opencensus/internal/ocmetric/metric.go	93.3% <100.0%> (-0.5%)	⬇️
metric/instrument/asyncfloat64.go	100.0% <100.0%> (ø)
metric/instrument/asyncint64.go	100.0% <100.0%> (ø)
metric/instrument/instrument.go	100.0% <100.0%> (ø)
metric/instrument/syncfloat64.go	100.0% <100.0%> (ø)
metric/instrument/syncint64.go	100.0% <100.0%> (ø)
... and 3 more