perf: add a reserve method to Injector to avoid repeated resizing

[bantonsson]

Changes

Adds a reserve method to Injector to hint at the number of elements that will be added to avoid multiple resize operations of the underlying data structure. Has an empty default implementation.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.7%. Comparing base (61f86c7) to head (d23925b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3173   +/-   ##
=====================================
  Coverage   80.7%   80.7%           
=====================================
  Files        126     126           
  Lines      22331   22397   +66     
=====================================
+ Hits       18023   18089   +66     
  Misses      4308    4308           

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[cijothomas]

Nice improvement!

(Were you seeing propagator alloc/resize showing up in profiling in real applications?)

[bantonsson]

Yes, it showed up in profiles of an internal service.

[lalitb]

@bantonsson - Good to merge once the conflict is resolved.