[Backport to 6.1] util/backtrace: Optimize formatter to reduce memory allocation overhead

[tchaikov]

This commit addresses a critical memory allocation issue in backtrace formatting by directly specializing fmt::formatter for backtrace types, eliminating dependency on iostream-based formatting.

Problem:
When Seastar applications experience high memory pressure, logging backtrace information could fail due to additional memory allocation required by iostream formatting. This resulted in errors like:

ERROR 2024-12-10 01:59:16,905 [shard 0:main] seastar_memory - seastar/src/core/memory.cc:2126 @void seastar::memory::maybe_dump_memory_diagnostics(size_t, bool): failed to log message: fmt='Failed to allocate {} bytes at {}': std::__ios_failure (error iostream:1, basic_ios::clear: iostream error)"

Solution:

• Implement direct fmt::formatter specialization for backtrace
• Remove reliance on operator<< for string representation
• Reduce memory allocation pressure during out-of-memory scenarios
• Improve reliability of backtrace logging under extreme memory constraints

Compatibility Improvements:

• Ensure consistent fmt::formatter availability across fmt library versions
• Deprecate two simple_backtrace constructors previously added in 095a07b, as they are no longer needed with fmt::join
• Maintain robust formatting approach independent of library version

Impact:
This change enhances system resilience by enabling backtrace logging even under significant memory pressure, providing more reliable post-mortem debugging information.

Signed-off-by: Kefu Chai kefu.chai@scylladb.com
(cherry picked from commit 3133ecd)

Parent PR: scylladb/seastar#2572
Refs scylladb/scylladb#22727

[denesb]

Compilation failed with g++, but we probably don't care about that in private release branches.

[denesb]

Closed by af8ae07.
Submodule update: scylladb/scylladb@b567d60.