Breakdown metrics warning after running for a while

[SylvainJuge]

Describe the bug

After running for a while, the agent continuously spam the standard output with this warning from the otel SDK:

WARN io.opentelemetry.sdk.metrics.internal.state.DefaultSynchronousMetricStorage - Instrument elastic.span_breakdown has exceeded the maximum allowed cardinality (1999)

The reason we have a high cardinality here is because we capture the span self time as a metric attribute to allow using it in ingest pipeline later.

Given the current state of breakdown metrics, we should probably:

• make the feature opt-in through configuration, which requires to migrate it first to autoconfig
• unplug it for now, but keep the code around for later use.
• remove the feature completely until its ready for prime-time