feat: Add counter metric for http.send network requests

[anivar]

Overview

This PR implements issue #6838 by adding a counter metric to track the number of actual HTTP network requests made by the http.send builtin.

What this adds

• New metric: rego_builtin_http_send_network_requests
• Purpose: Track actual HTTP network calls made during policy evaluation
• Scope: Counts only network requests, excludes cache hits

Implementation details

The counter increments in both cache implementations:

• interQueryCache.ExecuteHTTPRequest() - for inter-query cache misses
• intraQueryCache.ExecuteHTTPRequest() - for intra-query cache misses

This ensures all actual network requests are counted while cache hits are properly excluded.

Usage

When querying with ?metrics=true, the response will include:

{
  "metrics": {
    "counter_rego_builtin_http_send_network_requests": 3,
    "counter_rego_builtin_http_send_interquery_cache_hits": 1
  }
}

Testing

Added comprehensive test coverage in TestHTTPSendMetrics that verifies:

• Counter increments for successful requests
• Counter increments for failed requests
• Counter excludes cache hits
• Multiple requests increment correctly

Impact

This gives OPA users visibility into HTTP usage patterns in their policies, helping with:

• Performance monitoring
• Cost optimization
• Debugging HTTP-related issues

Fixes #6838

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[sspaink]

Thank you for implementing this, looks good.

Updating the documentation would be nice but perhaps that can be handled in a separate effort to resolve #6730

[anivar]

Thank you for implementing this, looks good.

Updating the documentation would be nice but perhaps that can be handled in a separate effort to resolve #6730

@sspaink Thank you for the review! I agree - will handle the documentation update in a separate PR to keep this one focused.

[charlieegan3]

Thanks for this @anivar, if you fancy taking a go at #6730 that'd be super helpful since you know how this bit works now.