feat: implement observability and metrics (Task 22)

- Add metrics package tracking cache hit-rate, API quota, circuit breaker trips, search time
- Integrate metrics with cache layer, yandex client, and API handlers
- Add /metrics HTTP endpoint for Prometheus-compatible metrics exposure
- Write tests for metrics functionality across cache, yandex, and API handlers
- Update test files to support new metrics infrastructure

internal/yandex/client.go

@@ -9,6 +9,8 @@ import (
 	"net/url"
 	"sync"
 	"time"
+
+	"trip-planner/internal/metrics"
 )
 
 // Client represents a Yandex Schedules API client with rate limiting,
@@ -19,6 +21,7 @@ type Client struct {
 	rateLimiter    *tokenBucket
 	circuitBreaker *circuitBreaker
 	retryConfig    *retryConfig
+	metrics        *metrics.Metrics
 }
 
 // tokenBucket implements a token bucket rate limiter.
@@ -111,6 +114,13 @@ func WithRetryConfig(maxRetries int, baseBackoff, maxBackoff time.Duration, jitt
 	}
 }
 
+// WithMetrics sets the metrics recorder for the client.
+func WithMetrics(m *metrics.Metrics) Option {
+	return func(c *Client) {
+		c.metrics = m
+	}
+}
+
 // Do executes a Yandex API request with rate limiting, circuit breaking, and retry.
 func (c *Client) Do(ctx context.Context, method, path string, query map[string]string) (*Response, error) {
 	// Apply rate limiting
@@ -128,6 +138,7 @@ func (c *Client) Do(ctx context.Context, method, path string, query map[string]s
 	for attempt := 0; attempt <= c.retryConfig.maxRetries; attempt++ {
 		// Check circuit breaker on each retry attempt
 		if !c.circuitBreaker.allow() {
+			c.metrics.RecordCircuitBreakerTrip()
 			return nil, fmt.Errorf("circuit breaker is open")
 		}
 
@@ -155,6 +166,7 @@ func (c *Client) Do(ctx context.Context, method, path string, query map[string]s
 	}
 
 	c.circuitBreaker.recordFailure() // final failure
+	c.metrics.RecordCircuitBreakerTrip()
 	return nil, err
 }
 
@@ -248,7 +260,6 @@ func (e *APIError) Error() string {
 	return fmt.Sprintf("API error %d: %s", e.Code, e.Message)
 }
 
-// newAPIError creates an APIError from an HTTP response.
 func newAPIError(code int, message string) *APIError {
 	return &APIError{Code: code, Message: message}
 }
@@ -320,9 +331,6 @@ func newCircuitBreaker() *circuitBreaker {
 }
 
 
-// ResetCircuitBreaker resets the circuit breaker to its initial closed state.
-// This is useful for testing or recovery scenarios where the circuit needs to be
-// manually reset without waiting for the timeout.
 func (cb *circuitBreaker) ResetCircuitBreaker() {
 	cb.mu.Lock()
 	defer cb.mu.Unlock()
@@ -403,4 +411,4 @@ func applyJitter(backoff time.Duration) time.Duration {
 func randFloat64() float64 {
 	// Use math/rand with a seed based on function call index for variability
 	return rand.Float64()
-}
+}