Resolve signature ambiguity in `MetricsEmitter`

Coming from Unity and C# 9 I observed an
```
Ambiguous invocation.
Candidates are:
    EmitCounter(string, int, IEnumerable>?, Scope?)
    EmitCounter(string, int, ReadOnlySpan>, Scope?)
    all from class SentryMetricEmitter
```

Based on [The History of C#](https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history) this problem exists for projects using C# 13 (resolution priority) or older.

Start at the SentryMetricEmitter class and inspect the two EmitCounter overloads shown in the report, focusing on calls from projects using C# versions before 13. The work is done when those callers no longer receive an ambiguous-invocation diagnostic while the existing metric-emission behavior remains available.

Written by the indexing model from the issue text.