expfmt: prevent st@ leaking to Gauge and Untyped samples in OpenMetrics 2.0

[dashpole]

Some small follow-ups to the OpenMetrics 2.0 support introduced by #894.

Previously, writeOpenMetrics20Sample inspected metric.Counter.CreatedTimestamp directly when rendering samples. If a dto.Metric struct had multiple type fields populated (e.g., a Gauge with a leftover Counter.CreatedTimestamp), st@ would incorrectly render on Gauge or Untyped sample lines.
This PR:

• Passes startTimestamp explicitly to writeOpenMetrics20Sample only from the Counter branch (passing nil for Gauge and Untyped).
• Cleans up dead NaN/Inf branches in writeOpenMetrics20Timestamp in favor of direct strconv.AppendFloat formatting.
• Adds test cases verifying st@ is isolated to Counter metrics and does not leak onto Gauge/Untyped samples.

[Copilot]

Pull request overview

Follow-up fixes to the OpenMetrics 2.0 expfmt writer to ensure st@ (start timestamp) is emitted only for Counter samples, plus a small cleanup/adjustment around timestamp formatting and related test coverage.

Changes:

• Pass CreatedTimestamp explicitly only for Counter samples so st@ doesn’t leak into Gauge/Untyped output.
• Simplify writeOpenMetrics20Timestamp by relying on strconv.AppendFloat behavior directly.
• Improve proto timestamp formatting for pre-epoch timestamps and add coverage for those cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
expfmt/openmetrics_2_0_create.go	Ensures st@ is only written when explicitly provided (Counters), simplifies timestamp writing, and improves proto timestamp formatting for pre-epoch values.
expfmt/openmetrics_2_0_create_test.go	Adds scenarios preventing st@ leakage to Gauge/Untyped and adds tests for pre-epoch proto timestamp formatting; adjusts timestamp unit tests.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.