feat: selfmonitor metrics support dynamic labels and gc

[shunjiazhu]

Change-Id: I74af3156bbc37e047cfc7f7b6c26a807973aceb5

1. 自监控API，支持在打点时动态的传入TagValue，用于观测在插件初始化时无法确定label value的场景，比如http code等。

// ExportMetricRecords exports all metrics bound to this metric record.
// The results may be a list of map[string]string, each map[string]string is a set of measurements has the same labels.
// for example:
// []{
// {"counters":"{\"http_flusher_dropped_events\":\"3.0000\",\"http_flusher_flush_failure_count\":\"6.0000\",\"http_flusher_matched_events\":\"1.0000\",\"http_flusher_retry_count\":\"5.0000\",\"http_flusher_unmatched_events\":\"2.0000\"}","gauges":"{\"http_flusher_flush_latency_ns\":\"7.0000\"}","labels":"{\"PluginId\":\"13\",\"PluginType\":\"flusher_http\",\"RemoteURL\":\"http://localhost:8081\"}"}
// {"counters":"{\"http_flusher_status_code_count\":\"8.0000\"}","gauges":"{}","labels":"{\"PluginId\":\"13\",\"PluginType\":\"flusher_http\",\"RemoteURL\":\"http://localhost:8081\",\"status_code\":\"200\"}"}
// {"counters":"{\"http_flusher_status_code_count\":\"9.0000\"}","gauges":"{}","labels":"{\"PluginId\":\"13\",\"PluginType\":\"flusher_http\",\"RemoteURL\":\"http://localhost:8081\",\"status_code\":\"400\"}"}
// {"counters":"{\"http_flusher_error_count\":\"10.0000\"}","gauges":"{}","labels":"{\"PluginId\":\"13\",\"PluginType\":\"flusher_http\",\"RemoteURL\":\"http://localhost:8081\",\"level\":\"error\",\"reason\":\"timeout\"}"}
// {"counters":"{\"http_flusher_error_count\":\"11.0000\"}","gauges":"{}","labels":"{\"PluginId\":\"13\",\"PluginType\":\"flusher_http\",\"RemoteURL\":\"http://localhost:8081\",\"level\":\"warn\",\"reason\":\"retry\"}"}
// {"counters":"{\"http_flusher_error_count\":\"12.0000\"}","gauges":"{}","labels":"{\"PluginId\":\"13\",\"PluginType\":\"flusher_http\",\"RemoteURL\":\"http://localhost:8081\",\"level\":\"error\",\"reason\":\"dropped\"}"}
// }
// Note:
// A metric may have three levels of labels
// 1. MetricsRecord Level Const Labels, like PluginType=flusher_http, PluginId=1
// 2. Metric Level Const Labels, for example, flusher_http may have a const label: RemoteURL=http://aliyun.com/write
// 3. Metric Level Dynamic Labels, like status_code=200, status_code=204

2. 自监控支持序列过期，当一个指标的一个特定的Tags组合长期没有打点时，允许其gc掉，防止因为内存中常驻维度过高而产生的内存开销。

	cumulativeCounter := NewCumulativeCounterMetricVector("test", constLabels, getTagNames(allTags),
		WithExpiration(time.Hour),
		WithCardinalityLimit(1000),
	)

[Takuka0311]

百炼自动化审查：建议保持开启。

本 PR 为自监控模块引入动态标签、指标序列 GC 及 Histogram 支持，并修改了核心 API ExportMetricRecords 的返回值类型。当前存在合并冲突且涉及 API 签名变更，需维护者审查兼容性与合并策略，故保持开启。.

最佳落地路径：

建议作者先 rebase main 解决合并冲突。维护者需重点评估 ExportMetricRecords 签名变更对下游插件的向后兼容性、动态标签与 GC 机制的并发安全性及高基数内存控制。CI 通过且审查完成后可合入。.

已核对内容：

• merge_conflict: PR 当前 mergeable 状态为 false，存在合并冲突，需 rebase main。. (68a8d097e14f)
• api_breaking_change: ExportMetricRecords 返回值由 map[string]string 改为 []map[string]string，同步修改了多处调用方展开逻辑，属于核心自监控 API 签名变更。. (pkg/selfmonitor/metrics_record.go:48, 68a8d097e14f)
• feature_scope: 新增动态标签支持、指标过期 GC 机制及 Histogram 类型，涉及 pkg/selfmonitor 下多个核心实现文件与测试用例。. (pkg/selfmonitor/metrics_imp_v2.go, 68a8d097e14f)

百炼审查备注：模型 qwen3.6-max-preview；对照提交 7099f790b8a3。