fix: merge request context attributes with custom metrics

[daniyelnnr]

What is the purpose of this pull request?

Fixes an issue where custom metrics recorded by IO apps were missing request context attributes (account, route_id, status_code, etc.), resulting in incomplete metric data.

Implements automatic merging of request-scoped base attributes with custom attributes using OpenTelemetry Context API.

What problem is this solving?

Before: Custom metrics from apps only contained app-specific attributes:

{ "operation": "vm-global-css", "status": "success" }

After: Custom metrics automatically include request context:

{
  "vtex.account.name": "paypalstore",
  "component": "http-handler", 
  "route_id": "apiRouteHandler",
  "route_type": "public",
  "status_code": "200",
  "operation": "vm-global-css",
  "status": "success"
}

Key features:

• Base attributes set once per request via runWithBaseAttributes()
• Automatically merged with custom attributes in all metric calls
• Base attributes take precedence on key conflicts (transparent to apps)
• Custom attributes limited to 7, base attributes unlimited
• Zero breaking changes - apps continue using existing API

How should this be manually tested?

1. Deploy an app that records custom metrics using global.diagnosticsMetrics.recordLatency()
2. Verify metrics in data source include both custom attributes AND request context (account, route_id, status_code)
3. Confirm no duplicate or missing attributes

Screenshots or example usage

// From render-to-string app - no changes needed
global.diagnosticsMetrics?.recordLatency(elapsed, { 
  operation: 'my-operation',
  status: 'success' 
})

// Result automatically includes request context from middleware

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Requires change to documentation, which has been updated accordingly.

[silvadenisaraujo]

LGTM!