feat(callbacks): add native OpenTelemetry callback handler

[JopenChen]

What type of PR is this?

feat

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• This PR title match the format: (optional scope):
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level.

(Optional) Translate the PR title into Chinese.

feat(callbacks): 新增原生 OpenTelemetry callback handler

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
Add a new callbacks/otel package that implements callbacks.Handler with OpenTelemetry tracing following the GenAI semantic conventions.

Addresses community request #1028 for native OTel support. Currently users must implement callbacks manually to get distributed tracing for LLM applications built with eino.

Features:

• Span creation for ChatModel, Tool, Retriever, Embedding, Graph, Chain, and Workflow components
• GenAI standard attributes: gen_ai.system, gen_ai.operation.name, gen_ai.request.model, gen_ai.response.model, gen_ai.usage.input_tokens, gen_ai.usage.output_tokens, gen_ai.usage.total_tokens, gen_ai.tool.name
• Token usage extraction from model.CallbackOutput
• Query and document count attributes for Retriever components
• Nested span propagation (child spans automatically link to parent via context)
• Error recording with OpenTelemetry span status codes
• TimingChecker implementation - skips stream callbacks to avoid unnecessary stream copying overhead
• Configurable via WithTracerProvider and WithTracer options
• Uses OTel v1.21.0 to maintain Go 1.18 compatibility

Usage:

import (
    "github.com/cloudwego/eino/callbacks"
    "github.com/cloudwego/eino/callbacks/otel"
)

// Global registration
callbacks.AppendGlobalHandlers(otel.NewHandler())

// Or with custom TracerProvider
handler := otel.NewHandler(otel.WithTracerProvider(tp))

zh(optional):
新增 callbacks/otel 包，实现遵循 GenAI 语义约定的 OpenTelemetry callback handler。

响应社区需求 #1028，提供原生 OTel 支持。目前用户必须手动实现 callbacks 才能获得分布式追踪。

特性：

• 为 ChatModel、Tool、Retriever、Embedding、Graph、Chain、Workflow 组件创建 Span
• GenAI 标准属性：gen_ai.system、gen_ai.operation.name、gen_ai.request.model、gen_ai.response.model、gen_ai.usage.*、gen_ai.tool.name
• 从 model.CallbackOutput 提取 Token 使用量
• Retriever 组件的查询和文档数量属性
• 嵌套 Span 自动传播（子 Span 通过 context 自动关联父 Span）
• 错误记录与 OpenTelemetry Span 状态码
• 实现 TimingChecker——跳过 stream 回调以减少不必要的流拷贝开销
• 通过 WithTracerProvider 和 WithTracer 选项配置
• 使用 OTel v1.21.0 以保持 Go 1.18 兼容性

(Optional) Which issue(s) this PR fixes:

Fixes #1028

(optional) The PR that updates user documentation:

Will follow up with a docs PR if this feature is accepted.

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