fix(collector): use Telemetry API event time as metric timestamp

[dmedovich]

• Track the latest Telemetry API event time observed in recordMetrics on the receiver (lastEventTime).
• Use lastEventTime as the data point timestamp in flushMetricsLocked, falling back to time.Now() only when no event has been observed yet (in which case AppendDataPoints exports nothing anyway).
• Access is serialized by the existing r.mu, which already protects both recordMetrics and flushMetricsLocked.

This mirrors the behavior of the log pipeline and resolves the metric/log timestamp divergence described in the linked issue.

Testing

• Added TestMetricTimestampMatchesEventTime, which records events with a fixed RFC3339 time, performs a delayed flush, and asserts every Sum/Histogram data point's Timestamp() matches the event time rather than wall-clock time.Now().
• go test ./... and go vet ./... pass for the package.

Fixes #2263

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: dmedovich / name: Daniil Bee (a65afb2)

[dmedovich]

@wpessers

[wpessers]

Thank you for working on this @dmedovich! I will take some time tomorrow to take a closer look at your changes!