receive: Enable ExportCreatedMetric to preserve StartTimeUnixNano from OTLP

[stefanorth]

Summary

The OTLP handler in Thanos Receive does not export created timestamps (_created metrics) even though theotlptranslator supports this functionality via the ExportCreatedMetric setting. This causes start_time_unix_nano from OTLP cumulative counters to be discarded, preventing proper counter reset detection and rate calculations.

Problem Details

The settings initialization in the function convertToPrometheusFormat() (File: handler_otlp.go) is missing ExportCreatedMetric:

settings := otlptranslator.Settings{                                                                         
    AddMetricSuffixes:         true,
    DisableTargetInfo:         !h.options.OtlpEnableTargetInfo,
    PromoteResourceAttributes: h.options.OtlpResourceAttributes,
    // ExportCreatedMetric is not set (defaults to false)
}

Expected behavior:
When an OTLP cumulative counter with start_time_unix_nano is ingested, Thanos should create a corresponding _created metric (e.g., my_counter_total_created) containing the start timestamp.

Actual behavior:
The start_time_unix_nano field is silently discarded. Only the main counter metric is stored.

Impact

Without created timestamps:

• Counter resets cannot be detected properly
• Rate calculations may be incorrect

Proposed Solution

Enable created timestamp export in pkg/receive/handler_otlp.go:

settings := otlptranslator.Settings{
    AddMetricSuffixes:         true,
    DisableTargetInfo:         !h.options.OtlpEnableTargetInfo,
    PromoteResourceAttributes: h.options.OtlpResourceAttributes,
    ExportCreatedMetric:       true,  // ADD THIS LINE
}

This leverages existing functionality in the otlptranslator package without requiring additional code changes.