Add metrics collection support

[Nancy-Chauhan]

Feature Request\n\nWhile the SDK has a Metric model and can serialize/send metrics, there's no high-level API for collecting metrics from an application.\n\n## Proposed API\n\npython\nfrom acme_sdk.metrics import MetricsCollector, Counter, Histogram\n\ncollector = MetricsCollector(client=client)\n\n# Define metrics\nrequest_count = collector.counter(\n name="http.request.count",\n description="Total HTTP requests",\n tags={"service": "api"},\n)\n\nrequest_duration = collector.histogram(\n name="http.request.duration",\n description="Request duration in ms",\n buckets=[10, 50, 100, 250, 500, 1000],\n)\n\n# Record values\nrequest_count.increment()\nrequest_duration.record(42.5)\n\n# Auto-flush every 60 seconds\ncollector.start(flush_interval=60)\n\n\n## Requirements\n\n- Counter, Gauge, Histogram, Summary metric types\n- Thread-safe recording\n- Periodic background flush\n- Tags/labels support\n- Integration with existing exporters\n\nThis is a large feature that should be scoped carefully for v2.0.