rabbitmq_tracing: explicitly specify UTF-8 encoding for served trace files

[bas0N]

Set charset=utf-8 in Content-Type header to ensure proper display of Cyrillic and other Unicode characters in browsers.

Fixes #13952

Proposed Changes

This PR fixes incorrect encoding display for Cyrillic and other Unicode characters when viewing trace log files through the RabbitMQ management UI. The issue was that browsers were misinterpreting UTF-8 encoded content as Latin-1 when no charset was specified in the Content-Type header.

Why this change:

• Trace files are written in UTF-8 by rabbit_tracing_consumer (using io_lib:format and rabbit_json:encode)
• Without an explicit charset parameter, browsers default to Latin-1/ISO-8859-1, causing Cyrillic characters to display as garbled text (e.g., Привет мир → Привет мир)
• Adding charset=utf-8 to the Content-Type header ensures browsers correctly interpret the file content

Solution chosen:
The original issue suggested a client-side JavaScript solution, but we implemented a server-side fix instead because:

• Server-side solution provides a proper URL in the browser (the JS solution resulted in about:blank)
• More reliable and standard approach (charset should be specified in HTTP headers)
• Simpler and cleaner - no client-side workarounds needed
• Follows HTTP best practices (RFC 7231)

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue rabbitmq_tracing should open trace files using UTF-8 for encoding #13952)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

Put an x in the boxes that apply.
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This is simply a reminder of what we are going to look for before merging your code.

• Mandatory: I (or my employer/client) have have signed the CA (see https://github.com/rabbitmq/cla)
• I have read the CONTRIBUTING.md document
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

Changes made:

• Modified deps/rabbitmq_tracing/src/rabbit_tracing_wm_file.erl to set Content-Type: text/plain; charset=utf-8 header in serve/2
• Added test trace_file_content_type_test in deps/rabbitmq_tracing/test/rabbit_tracing_SUITE.erl to verify the Content-Type header includes charset

Why this is safe:

• Content negotiation remains unchanged (still accepts text/plain)
• Charset is optional metadata per RFC 7231 - adding it doesn't break compatibility
• Trace files are consistently written as UTF-8 by the plugin
• Similar pattern exists elsewhere in codebase (e.g., rabbit_mgmt_oauth_bootstrap.erl sets text/javascript; charset=utf-8)

Testing:

• Added test case that verifies Content-Type header contains both text/plain and charset=utf-8
• Manually tested with Cyrillic characters - displays correctly in browsers
• Existing tests continue to pass

Before the fix:

After the fix:

[michaelklishin]

Thank you for contributing, @bas0N.