Adds Send + Sync to InstrumentCore (metrics)

[markdingram]

• removes superfluous declarations on derived traits

as suggested: #422 (review)

[codecov]

Codecov Report

Merging #423 (1913a5a) into master (f00c4c2) will decrease coverage by 0.01%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
- Coverage   48.34%   48.32%   -0.02%     
==========================================
  Files          65       65              
  Lines        5502     5502              
==========================================
- Hits         2660     2659       -1     
- Misses       2842     2843       +1     

Impacted Files	Coverage Δ
opentelemetry/src/metrics/meter.rs	50.00% <ø> (ø)
opentelemetry/src/metrics/registry.rs	0.00% <ø> (ø)
opentelemetry/src/sdk/metrics/mod.rs	0.00% <0.00%> (ø)
opentelemetry/src/metrics/noop.rs	20.00% <50.00%> (ø)
...ntelemetry/src/sdk/metrics/aggregators/ddsketch.rs	76.76% <0.00%> (-0.20%)	⬇️

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[TommyCpp]

Thanks for working on this!