Do not pass the formatted message into span_name

[petyosi]

Fixes #64

[Copilot]

Pull Request Overview

This PR fixes an issue where formatted messages were being incorrectly used as span names instead of the original message templates. The change ensures that span names remain as the original template format while the formatted message is stored as a separate attribute.

• Updated logfireFormatWithExtras to return an object instead of a tuple for better destructuring
• Changed span creation to use msgTemplate as the span name instead of formattedMessage
• Added ATTRIBUTES_MESSAGE_KEY to store the formatted message as an attribute

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/logfire-api/src/index.ts	Updated span creation functions to use template as span name and store formatted message as attribute
packages/logfire-api/src/formatter.ts	Refactored return type from tuple to object for better readability
packages/logfire-api/src/constants.ts	Added new constant for message attribute key
.changeset/cyan-bats-jump.md	Added changeset entry for the fix

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