feat(otel): add UseMonotonicCounters option for correct Prometheus counter type

[apridosimarmata]

Problem

The OpenTelemetry metrics handler uses Int64UpDownCounter for all counters. This causes Prometheus to report Temporal counters (like workflow_completed) as gauges instead of counters with the _total suffix, since Int64UpDownCounter is non-monotonic.

See #2140 for details.

Solution

Added a UseMonotonicCounters option to MetricsHandlerOptions. When set to true, the handler creates counters using Int64Counter (monotonic) instead of Int64UpDownCounter.

The default remains false for backward compatibility, as discussed in the issue thread.

Usage

handler := opentelemetry.NewMetricsHandler(opentelemetry.MetricsHandlerOptions{
    Meter:                meterProvider.Meter("temporal-sdk-go"),
    UseMonotonicCounters: true,
})

Testing

Added TestMonotonicCounterHandler that verifies counters are reported with IsMonotonic: true when the option is enabled. All existing tests continue to pass.

Fixes #2140

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