Use relaxed ordering for external stream counters

[BharatDeva]

PR checklist:

• Did you run ClangFormat? Not available in this local environment; this header-only change keeps the existing one-line method style and git diff --check is clean.
• Did you separate headers to a different section in existing community code base? Not applicable, no new headers were added.
• Did you surround proton: starts/ends for new code in existing community code base? Not applicable, this updates Proton-owned code only.

Please write user-readable short description of the changes:

Fixes #488.

ExternalStreamCounter is used for metrics counters. The reset/read methods already use std::memory_order_relaxed, but add, set, and getCounters() snapshot operations were still using the default sequentially consistent atomic operations.

This PR makes those counter-only operations explicit relaxed atomics as well:

• fetch_add(..., std::memory_order_relaxed) for counter increments
• store(..., std::memory_order_relaxed) for setter methods
• load(std::memory_order_relaxed) for the metrics snapshot map

Validation performed locally:

git diff --check
# clean

g++ -std=c++20 -Isrc -Ibase -c /mnt/c/Users/bhara/AppData/Local/Temp/proton_counter_check.cpp -o /tmp/proton_counter_check.o
# passed

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[yokofly]

@codex review

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[yokofly]

// edit v1: assign->check

@BharatDeva
Thanks, LGTM.
The change is trivial. Can you check the CLA? as #1176 (comment)