feat: Add tracing spans for database operations

[uuushiro]

Summary

This PR adds optional tracing instrumentation for database operations, enabling distributed tracing and observability.

• Introduces tracing-spans feature flag for opt-in tracing support
• Creates spans for all database operations (execute, query_one, query_all) and transaction flows (begin, commit, rollback)
• Records OpenTelemetry-compatible semantic attributes (db.system, db.operation, db.statement, otel.status_code)
• Uses .instrument(span) pattern for proper async span context propagation

Motivation

As noted in #2867, logs alone make it difficult to understand database call hierarchies and timing within request contexts. Spans provide:

• Hierarchy: Which DB calls happened under which request
• Timing: Precise duration of each operation
• Context: Error status and query details for debugging

Design Decision

The original issue proposed an ObservabilityHook pattern. This PR takes a simpler approach by using the tracing crate directly:

1. Standard ecosystem: tracing is the de facto standard for Rust instrumentation
2. Simpler integration: Users just enable the feature and configure their tracing subscriber
3. Backend agnostic: Works with any tracing-compatible backend (Jaeger, Datadog, OpenTelemetry, etc.)
4. SQL safety: Relies on tracing backends (e.g., Datadog Agent) for SQL obfuscation rather than implementing custom filtering

Usage

[dependencies]
sea-orm = { version = "2.0", features = ["tracing-spans"] }

// Set up any tracing subscriber
tracing_subscriber::fmt()
    .with_max_level(tracing::Level::INFO)
    .init();

// All database operations automatically generate spans
let db = Database::connect("postgres://...").await?;
let users = User::find().all(&db).await?;  // Generates span with db.* attributes

Span Attributes

Attribute	Description	Example
db.system	Database type	postgresql, mysql, sqlite
db.operation	SQL operation	SELECT, INSERT, UPDATE, DELETE
db.statement	SQL query	SELECT * FROM users WHERE id = $1
otel.status_code	Result status	OK, ERROR
exception.message	Error details	(on failure)

Closes #2867

[uuushiro]

#2867 (comment)