outputs.opentelemetry: add metric type overrides option as in outputs.prometheus_client

### Use Case

Opentelemetry output should support metric type override config (as prometheus_client), to ensure custom metrics are sent with correct OTLP type.

### Expected behavior

Metric type should be set according to the configuration e.g.
```
[outputs.opentelemetry.metric_types]
counter = [ "my_custom_counter_metric" ]
gauge = []
```

### Actual behavior

No support of setting OTLP metric type on output plugin level.

### Additional info

_No response_

Start by comparing the outputs.opentelemetry implementation with outputs.prometheus_client, focusing on how metric_types configuration is declared, parsed, and applied. Add equivalent counter and gauge override support for custom metrics, then verify that configured metrics are emitted with the expected OTLP types.

Written by the indexing model from the issue text.