docs: add Prometheus metrics env vars to .env.example

Add comprehensive documentation for Prometheus metrics configuration
variables to .env.example:

- ENABLE_METRICS: Toggle metrics collection (default: true)
- METRICS_EXCLUDED_HANDLERS: Regex patterns for endpoint exclusion
- METRICS_NAMESPACE: Metrics name prefix (default: "default")
- METRICS_SUBSYSTEM: Secondary metrics prefix
- METRICS_CUSTOM_LABELS: Static labels for app_info gauge

Includes examples, security warnings about high-cardinality labels,
and formatting consistent with existing configuration sections.

Related to PR #1313

Signed-off-by: Mihai Criveti <[email protected]>

Author: crivetimihai

.env.example

@@ -765,6 +765,45 @@ OTEL_BSP_MAX_QUEUE_SIZE=2048
 OTEL_BSP_MAX_EXPORT_BATCH_SIZE=512
 OTEL_BSP_SCHEDULE_DELAY=5000
 
+# Prometheus Metrics Configuration
+# Enable Prometheus-compatible metrics exposition for monitoring and alerting
+# Options: true (default), false
+# When true: Exposes metrics at /metrics/prometheus in Prometheus format
+# When false: Returns HTTP 503 on metrics endpoint
+ENABLE_METRICS=true
+
+# Comma-separated regex patterns for endpoints to exclude from metrics collection
+# Use this to avoid high-cardinality issues with dynamic paths or reduce overhead
+# Examples:
+# - Exclude SSE endpoints: /servers/.*/sse
+# - Exclude static files: /static/.*
+# - Exclude health checks: .*health.*
+# - Multiple patterns: /servers/.*/sse,/static/.*,.*health.*
+# Default: "" (no exclusions)
+METRICS_EXCLUDED_HANDLERS=
+
+# Prometheus metrics namespace (prefix for all metric names)
+# Used to group metrics by application or organization
+# Example: mycompany_gateway_http_requests_total
+# Default: "default"
+METRICS_NAMESPACE=default
+
+# Prometheus metrics subsystem (secondary prefix for metric names)
+# Used for further categorization within namespace
+# Example: mycompany_api_http_requests_total (if subsystem=api)
+# Default: "" (no subsystem)
+METRICS_SUBSYSTEM=
+
+# Custom static labels for app_info gauge metric
+# Format: comma-separated "key=value" pairs (low-cardinality values only)
+# WARNING: Never use high-cardinality values (user IDs, request IDs, timestamps)
+# Examples:
+# - Single label: environment=production
+# - Multiple labels: environment=production,region=us-east-1,team=platform
+# - K8s example: cluster=prod-us-east,namespace=mcp-gateway
+# Default: "" (no custom labels)
+METRICS_CUSTOM_LABELS=
+
 # Plugin Framework Configuration
 # Enable the plugin system for extending gateway functionality
 # Options: true, false (default)