[frontend] - add span links on synthetic requests

[puckpuck]

Related to #329

Changes

• Creates a new trace if the inbound request has a synthetic_request=true baggage (from load generator)
• The new trace is linked to the inbound load generator trace using span links

• Appropriate CHANGELOG.md updated for non-trivial changes

Depends on #331

[xoscar]

Hey @puckpuck this is looking great! I left a small non-blocking comment

[julianocosta89]

It looks good to me, I'd just love to do a dry run before merging it

[puckpuck]

I put together this branch: https://github.com/honeycombio/opentelemetry-demo/tree/baggage-end-to-end which combines #331 and #332

1. After starting it, go to Jaeger UI, and look for traces to the paymentservice (might take a minute before one appears)
2. This trace will have a app.synthetic_requests=true attribute on the frontend root span, and a app.payment.charged=false attribute on the paymentservice "charge" span. (also note the trace root has a link to the load generator trace).
3. Stop just the load generator. From a new terminal window run docker compose stop loadgenerator
4. Using your browser go to the demo app UI, and place an order for a product
5. Return to Jaeger UI, and look for the trace you just created with the paymentservice
6. This trace will have a root span that comes from your browser telemetry.sdk.language=webjs. There will be no attribute on this span or the subsequent frontend span about a synthetic request.
7. The paymentservice "charge" span will have app.payment.charge=true attribute for orders placed using a web browser.

[austinlparker]

@puckpuck can you resolve conflicts?