[Tracing] We need to be able to add attributes after span creation

[lmolkova]

See #23268 (comment).

Some information is available in response (like response properties, error codes, etc) and should be provided after span creation.

[kjg]

In #23652, we added some attributes after span creation by using

span := c.internal.Tracer().SpanFromContext(ctx)
span.SetAttributes([]tracing.Attribute{
	{Key: "db.cosmosdb.request_charge", Value: res.RequestCharge
})

However there might still be the opportunity for a nicer way to get access to a span than via the context.