Fix observable updown counter aggregation temporality in Geneva exporter

[Copilot]

Observable updown counters were incorrectly exporting delta values instead of cumulative values to Geneva. This happened because the GetAggregationTemporality method in the Geneva exporter only checked for kUpDownCounter instrument types but missed kObservableUpDownCounter types.

Both regular and observable updown counters should use cumulative aggregation temporality for proper Geneva export behavior, as updown counters represent values that can go up and down and need to maintain their absolute state rather than just reporting changes.

Changes

Updated the conditional check in GetAggregationTemporality to include both instrument types:

// Before
if (instrument_type == sdk::metrics::InstrumentType::kUpDownCounter)

// After  
if (instrument_type == sdk::metrics::InstrumentType::kUpDownCounter ||
    instrument_type == sdk::metrics::InstrumentType::kObservableUpDownCounter)

Added comprehensive unit test GetAggregationTemporalityTest that verifies:

• kUpDownCounter returns kCumulative (existing behavior)
• kObservableUpDownCounter returns kCumulative (fixed behavior)
• Other instrument types like kCounter and kHistogram continue to return kDelta

This minimal change ensures observable updown counters export their cumulative values to Geneva correctly, matching the behavior of regular updown counters.

Fixes #570.

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[cijothomas]

nit: add changelog too