Improve Span Links API

[Kludex]

Currently, it's not very ergonomic to create a span link.

Let's assume we have 2 services:

Main Service

from typing import Any

from opentelemetry import trace
from opentelemetry.propagate import extract
from pydantic import TypeAdapter
from redis import Redis

import logfire
import logfire.propagate

logfire.configure(service_name='subscriber')
logfire.instrument_redis()

TA = TypeAdapter[Any](Any)


def main():
    client = Redis()
    logfire.info('Subscriber is running')

    def watch_key(key: str):
        pubsub = client.pubsub()
        pubsub.subscribe(key)

        for message in pubsub.listen():
            if message['type'] == 'message':
                with logfire.span('process_message'):
                    data = TA.validate_json(message['data'])
                    context = extract(data.pop('headers', {}))
                    span = next(iter(context.values()))
                    with logfire.span('process_data', _links=[(span.get_span_context(), None)]):
                        logfire.info(f'Received data: {data}')

    watch_key('key')


main()

And...

Service A

from typing import Any

from pydantic import TypeAdapter
from redis import Redis

import logfire
import logfire.propagate

logfire.configure(service_name='service_a')
logfire.instrument_redis()

TA = TypeAdapter[Any](Any)

client = Redis()


@logfire.instrument()
def service_a():
    logfire.info('Service A is running')

    data: dict[str, Any] = {'data': 'Hello from Service A', 'headers': logfire.propagate.get_context()}
    client.publish('key', TA.dump_json(data))


service_a()

As you see, in the code, we set the headers with logfire.propagate.get_context(), and then on the subscriber, we need to run:

context = extract(data.pop('headers', {}))
span = next(iter(context.values()))
with logfire.span('process_data', _links=[(span.get_span_context(), None)]):
    ...

We can for sure improve this. Either by passing SpanContext to _links, or a Link.

This PR proposes creating:

1. logfire.propagate.build_span_link: Creates a span link from a ContextCarrier, so it makes easier to use the propagate API we created. But... We can also build a span_context, instead of Link itself. It may be a bit more useful. I'm open to rework this.
2. Adds Link and SpanContext to the _links parameter.

---

I need to add tests here. I've first implemented this to bring back the discussion.

• Closes Support for Span link with good DX interface and demo presentation #158

[codecov]

Codecov Report

Attention: Patch coverage is 56.25000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 99.93%. Comparing base (8434258) to head (16e5534).

Files with missing lines	Patch %	Lines
logfire/_internal/main.py	60.00%	2 Missing and 2 partials ⚠️
logfire/propagate.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #726      +/-   ##
===========================================
- Coverage   100.00%   99.93%   -0.07%     
===========================================
  Files          139      139              
  Lines        11206    11218      +12     
  Branches      1572     1574       +2     
===========================================
+ Hits         11206    11211       +5     
- Misses           0        5       +5     
- Partials         0        2       +2     

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