Reserve vector capacity for tempTagMessages in TLog commit path

[dlambrig]

Pre-reserve capacity for the tempTagMessages vector in commitMessages() to eliminate dynamic reallocations during message parsing. The tempTagMessages vector grows dynamically as messages are parsed, causing multiple reallocations in the hot commit path. Each reallocation involves allocating new memory, copying existing elements, and freeing old memory. This happens on every TLog commit, adding unnecessary overhead.

The PR estimates the number of messages based on total byte size and pre-reserve vector capacity:

size_t estimatedMsgCount = std::max(size_t(messages.size() / 150), size_t(10));
estimatedMsgCount = std::min(estimatedMsgCount, size_t(5000));
self->tempTagMessages.reserve(estimatedMsgCount);

• 150 bytes average message size
• Minimum reserve of 10 messages
• Cap at 5000 messages to prevent over-allocation

A microbenchmark was written. Each tests simulate the commitMessages() parsing loop by creating realistic TagsAndMessage objects with variable sizes and pushing them into a vector. The benchmark measures three strategies: NoReserve (current implementation with dynamic growth), WithReserve (optimization using size estimation), and ExactReserve (theoretical "best case" with perfect knowledge of message count). Test scenarios cover typical TLog workloads from light (10 messages) to burst (5000 messages) loads.

Benchmark	Time (ns)	CPU (ns)	items_per_second	% Improvement
NoReserve/10/100	3571	3553	2.81M/s	baseline
WithReserve/10/100	2351	2327	4.30M/s	+52.7% faster
ExactReserve/10/100	2430	2397	4.17M/s	+48.2% faster
NoReserve/100/150	13466	13376	7.48M/s	baseline
WithReserve/100/150	10334	10228	9.78M/s	+30.8% faster
ExactReserve/100/150	9292	9162	10.9M/s	+46.0% faster
NoReserve/1000/200	148803	148503	6.73M/s	baseline
WithReserve/1000/200	72418	72109	13.9M/s	+106.0% faster
ExactReserve/1000/200	72804	72457	13.8M/s	+105.0% faster
NoReserve/5000/150	797027	796247	6.28M/s	baseline
WithReserve/5000/150	566643	566094	8.83M/s	+40.7% faster
ExactReserve/5000/150	477830	477121	10.5M/s	+66.9% faster

Tests on playstation will be posted when completed.

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[foundationdb-ci]

Result of foundationdb-pr-clang-ide on Linux RHEL 9

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Result of foundationdb-pr-clang on Linux RHEL 9

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Result of foundationdb-pr on Linux RHEL 9

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