opentelemetry: metrics: preserve JSON asInt precision and metadata

  Fix two JSON metrics decode issues in flb_opentelemetry_metrics.c.

  - Parse `asInt` datapoints as exact int64 instead of double conversion.
  - Set gauge/counter samples with typed cmetrics setters to avoid >2^53
    rounding loss.
  - Keep counter reset behavior consistent by clamping negative int values
    when reset is not allowed.
  - Preserve resource/scope metadata from JSON payloads by cloning:
    - resource metadata/attributes
    - resource_metrics schema_url
    - scope metadata/attributes
    - scope_metrics schema_url
    into `context->external_metadata` for OTLP re-encoding

Signed-off-by: Eduardo Silva <eduardo@chronosphere.io>