chore: Expose metrics as separate file export

[just-boris]

Issue #, if available:

Description of changes:

Reduce loaded size when importing metrics

import { Metrics } from '@cloudscape-design/component-toolkit/internal'; // 3.8 Kb
import { Metrics } from '@cloudscape-design/component-toolkit/internal/metrics'; // 0.7 Kb

Why is this happening – @cloudscape-design/component-toolkit/internal also loads ResizeObserver polyfill which is a side effect, but side effects do not tree shake

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.22%. Comparing base (e7584f4) to head (b55d2ba).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #106   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          27       27           
  Lines         642      642           
  Branches      169      172    +3     
=======================================
  Hits          637      637           
  Misses          5        5           

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