Use write_concurrency=auto for better scalability

[NelsonVides]

Benchmarks generally show better scalability as the parallelism increases, on my 12CPU hardware it is already visible:

auto is available since OTP25, so we're good to go version-wise.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/prometheus_sup.erl	77.41% <ø> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[DenysGonchar]

looks good