ServiceContext lost when eager loading parent models

[Iipin]

Describe the bug

Hello 👋 I've stumbled upon a behavior that may be not a bug per se but was a bit unexpected, at least for me. I'd like to instrument my Vapor application for OpenTelemetry (in accordance with https://github.com/apple/swift-distributed-tracing) and for that reason wanted to create a new OT span for each database query. My approach was to decorate Fluent's Database:

import Fluent
import Tracing  // from https://github.com/apple/swift-distributed-tracing

struct TracedDatabase: Database {
    private let db: any Database

    // MARK: - Database

    public func execute(
        query: DatabaseQuery,
        onOutput: @escaping (DatabaseOutput) -> ()
    ) -> EventLoopFuture<Void> {
        let span = startSpan(query.description, ofKind: .client)
        let result = db.execute(query: query, onOutput: onOutput)
        result.whenComplete { _ in span.end() }
        return result
    }

    /* ... */
}

I could then do e.g. let myTracedDatabase = TracedDatabase(application.db) and use it as usual.
However, when performing MyModel.query(on: myTracedDatabase).with(.\$someParent).all() I could observe that:

• the first database query's span got properly attached to the same trace as request's root span,
• each of the following queries for MyModel's parents (.someParent in this case) got its own, new trace.

In other words, looking at https://github.com/apple/swift-distributed-tracing/blob/61c272fe72c9728c85952f0c6170cf491ac61dc0/Sources/Tracing/Tracer.swift#L47: parent model queries are executed with a different service context (so within separate tasks?) than the initial one.

As I said before, this might be an expected behavior which would be fine too – it's easy enough to work around this by storing the context object beforehand and passing it to startSpan explicitly.

To Reproduce

1. Install https://github.com/apple/swift-service-context,
2. Create a model ModelA with a parent model .modelB: ModelB,
3. Create a decorated database class; a slightly more compact example would be:

struct TracedDatabase: Database {
    let db: any Database

    // MARK: - Database

    public func execute(
        query: DatabaseQuery,
        onOutput: @escaping (DatabaseOutput) -> ()
    ) -> EventLoopFuture<Void> {
        print("current service context: \(ServiceContext.current)")
        return db.execute(query: query, onOutput: onOutput)
    }

    /* ... */
}

4. await ModelA.query(on: TracedDatabase(req.db)).with(\.$modelB).all(), inspect the output.

Expected behavior

The same service context object is printed twice.

Environment

• Vapor Framework version: 4.92.1
• Vapor Toolbox version: n/a
• OS version: swift:5.9 Docker image

The app is running with PostgreSQL 16 database, with fluent-postgres-driver version 2.8.0.

Additional context

Add any other context about the problem here.