Move ValueMap to mod file to allow for code reuse

[utpilla]

After #2004, we can potentially reuse ValueMap to track the measurements for any instrument. ValueMap offers a more performant tracking mechanism:

• Lesser contention (Read-Write locks instead of Mutexes)
• Allocation free hot-path (Switching to ValueMap's update logic would also help avoid unnecessary allocation of AttributeSet on the hot-path)

Currently, only counters (Sum aggreagtion) make use of ValueMap.

Changes

• Moving ValueMap to mod.rs (a common place from where other instruments can also use it)

Next

• I would be working on incorporating ValueMap for Gauge.

[codecov]

Codecov Report

Attention: Patch coverage is 89.23077% with 7 lines in your changes missing coverage. Please review.

Project coverage is 75.1%. Comparing base (ed82d78) to head (cdf80aa).

Files	Patch %	Lines
opentelemetry-sdk/src/metrics/internal/mod.rs	89.2%	7 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2012   +/-   ##
=====================================
  Coverage   75.1%   75.1%           
=====================================
  Files        122     122           
  Lines      20887   20887           
=====================================
+ Hits       15701   15702    +1     
+ Misses      5186    5185    -1     

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