Add fga-client.request.count metric for tracking HTTP requests

[SoulPancake]

Checklist

• I agree to the terms within the OpenFGA Code of Conduct.

Describe the problem you'd like to have solved

The Go SDK is missing the fga-client.request.count metric. The .NET SDK already emits this per-HTTP-request counter.

Describe the ideal solution

Add fga-client.request.count, incrementing once per HTTP request sent by the SDK, matching the .NET SDK implementation.

Alternatives and current workarounds

No response

References

• Closes (supersedes) Report a per-http call metric #126
• Reference : feat: report a per http call metric dotnet-sdk#173 (review)
• PR disabling duration metric by default: fix: disable http req duration metric by default #281
• Upstream tracking: Report a per-http call metric sdk-generator#418

Additional context

This will ensure cross-SDK parity with metrics and make it easier for users to monitor and understand SDK behavior across languages.