Update spec compliance matrix for C++ (metrics instrument names)

[marcalff]

Changes

Updated the following entry in the spec compliance matrix:

• Section Metrics, item "Instrument names conform to the specified syntax."
• Language C++

The lastest spec changes (#3648, #3422) are implemented by the following PR:

• [SDK] Increase metric name maximum length from 63 to 255 characters opentelemetry-cpp#2281
• [SDK] Allow metric instrument names to contain / characters opentelemetry-cpp#2303

so C++ can be marked compliant with the specification.

For non-trivial changes, follow the change proposal process.

• Related issues #
• Related OTEP(s) #
• Links to the prototypes (when adding or changing features)
• CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

[marcalff]

cc @open-telemetry/cpp-maintainers

[carlosalberto]

Enough reviews from C++ approvers, merging.