Feature/prometheus middleware

[HeerakKashyap]

Overview

This PR implements Prometheus instrumentation middleware for the GoFlash framework, addressing issue #37. The middleware provides comprehensive HTTP request metrics collection and exposes them via a configurable metrics endpoint.

Features

Core Metrics Collection

• Request Count: Total number of HTTP requests by method, path, and status code
• Request Duration: Histogram of request processing times
• Request Size: Size of incoming request bodies
• Response Size: Size of outgoing response bodies
• In-Flight Requests: Current number of requests being processed

Configuration Options

• Custom Namespace/Subsystem: Configure metric naming (e.g., myapp_api_requests_total)
• Configurable Metrics Path: Default /metrics, customizable via config
• Optional Metrics Endpoint: Can disable HTTP exposure for programmatic access only
• Custom Labels: Add application-specific labels to all metrics

Usage Examples

Basic Usage

app := flash.New()

// Apply with default configuration
app.Use(middleware.Prometheus())

// Or use the convenient mount function
middleware.MountPrometheus(app)

Implementation Details

Architecture

• Separate Registries: Each configuration uses its own Prometheus registry to avoid metric conflicts
• Response Writer Wrapping: Captures response size without affecting performance
• Thread-Safe: All metrics operations are thread-safe for concurrent requests
• Memory Efficient: Uses Prometheus's built-in memory management

Performance Considerations

• Minimal Overhead: Metrics collection adds minimal latency to request processing
• Efficient Labeling: Uses method and path labels for detailed breakdowns
• Configurable Buckets: Histogram buckets optimized for typical web request durations

Testing

The implementation includes comprehensive tests covering:

• Basic metrics collection and exposure
• Custom namespace/subsystem configuration
• Metrics endpoint availability and content
• Middleware behavior without metrics endpoint
• Request tracking across different HTTP methods and status codes

Security Considerations

• Metrics Endpoint: Should be protected in production environments
• Path Exposure: Request paths are included in metrics - consider sanitization for sensitive routes
• Access Control: Recommend using authentication/authorization for metrics endpoints

Dependencies

Added github.com/prometheus/client_golang/prometheus for metrics collection and exposure.

Closes #37

[codecov]

Codecov Report

❌ Patch coverage is 78.96104% with 81 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
middleware/pprof.go	40.69%	51 Missing ⚠️
ctx/ctx.go	83.06%	21 Missing ⚠️
middleware/prometheus.go	94.85%	8 Missing and 1 partial ⚠️

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