Relax instrument name validation per opentelemetry-specification#5092

[cijothomas]

Draft — depends on opentelemetry-specification#5092. Will be ready for review once the spec PR is merged.

Description

Relaxes the SDK instrument-name validation to match the proposed expanded character set in opentelemetry-specification#5092:

• Adds :, \, (, ), %, *, #, and space to the allowed character set.
• Drops the requirement that the first character be alphabetic.
• Space is restricted to non-leading and non-trailing positions.
• ASCII-only, case-insensitive identity, and the 255-character maximum are unchanged.

This enables Windows performance counter style names to be used directly as instrument names without workarounds — e.g. \Processor(_Total)\% Processor Time, \.NET CLR Memory(*)\# Bytes in all Heaps.

Backwards compatibility

Strict superset of the existing rule. Every name valid under the old regex remains valid; some previously-rejected names are now accepted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.93%. Comparing base (b0d5e79) to head (3cddecb).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7297      +/-   ##
==========================================
+ Coverage   89.85%   89.93%   +0.07%     
==========================================
  Files         273      273              
  Lines       13773    13773              
==========================================
+ Hits        12376    12387      +11     
+ Misses       1397     1386      -11     

Flag	Coverage Δ
unittests-Project-Experimental	89.74% <ø> (-0.06%)	⬇️
unittests-Project-Stable	89.81% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...lemetry/Metrics/Builder/MeterProviderBuilderSdk.cs	100.00% <ø> (ø)

... and 3 files with indirect coverage changes

[cijothomas]

Closing — open-telemetry/opentelemetry-specification#5092 was closed in favor of using Views for these cases (per spec clarification open-telemetry/opentelemetry-specification#5094). The Meter-level instrument name relaxation is no longer needed. The View-level fix is tracked in #7300.