chore: Fix CounterTest created timestamp flake

[zeitlinger]

What changed

Relaxed the CounterTest.testTotalStrippedFromName protobuf text assertion so created_timestamp accepts both forms emitted by protobuf text format:

• created_timestamp { seconds: ... nanos: ... }
• created_timestamp { seconds: ... }

Why

This test was flaky when the created timestamp landed exactly on a whole-second boundary. In that case, protobuf text format omits the nanos field when it is zero, but the test required nanos to always be present.

Impact

This does not change product behavior. It removes a nondeterministic test failure in CI by matching the actual protobuf output contract more accurately.

Root cause

The assertion encoded a stronger formatting assumption than protobuf guarantees. protobuf-java may omit zero-valued nanos in text output.

Validation

• ./mvnw test -pl prometheus-metrics-core -Dtest=CounterTest#testTotalStrippedFromName -Dcoverage.skip=true -Dcheckstyle.skip=true
• mise run build