fix(pika): skip already-decorated callbacks in _instrument_channel_consumers

[axelray-dev]

Description

When multiple basic_consume() calls are made on a single pika channel, each message produces duplicate (nested) CONSUMER spans -- N spans per message where N equals the number of consumers registered on the channel.

The root cause is that _instrument_channel_consumers() re-wraps every callback on each basic_consume() call without checking whether the callback is already decorated. The _original_callback attribute set on the decorated callback accumulates nested wrappers.

This adds a guard in _instrument_channel_consumers() to skip callbacks that already have an _original_callback attribute, indicating they were wrapped in a prior call.

Addresses #4667

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Added a regression test test_instrument_consumers_skips_already_decorated_callbacks that:

1. Creates a channel with 2 consumer infos, each with a real callback function
2. Calls _instrument_channel_consumers() twice (simulating two basic_consume() calls)
3. Asserts _decorate_callback is called exactly 2 times (once per consumer, not 4)
4. Asserts each callback's _original_callback still points to the original function

• Test added

Does This PR Require a Core Repo Change?

• Yes.
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: axelray-dev / name: axelray-dev (ee766cb)