Fix some bugs in Runtime metrics

[xiang17]

Fixes #383.

Changes

• For gc.fragmentation.ratio, use ObservableGauge and change value from percentage to fraction in range of [0, 1]
• Rename alloc.rate to gc.allocated.bytes
• Updates units to use Bytes for data volume; use nanoseconds in Int64 rather than seconds or milliseconds in Double
• gc.committed: use ObservableGauge
• Update all value types from Int32 to Int64

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue Bringing .NET Runtime metrics instrumentation to Stable #335

[codecov]

Codecov Report

Merging #409 (e3ae6a2) into main (768d94b) will not change coverage.
The diff coverage is 0.00%.

❗ Current head e3ae6a2 differs from pull request most recent head 50002c4. Consider uploading reports for the commit 50002c4 to get more accurate results

@@          Coverage Diff          @@
##            main    #409   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        155     155           
  Lines       4776    4776           
=====================================
  Misses      4776    4776           

Impacted Files	Coverage Δ
...elemetry.Instrumentation.Runtime/RuntimeMetrics.cs	0.00% <0.00%> (ø)