[o11y] Publish go-elasticsearch transport metrics through OTel measures

[inge4pres]

Describe the current state

The docappender Elasticsearch client used to perform HTTP requests has built-in support for some metrics (see https://github.com/elastic/elastic-transport-go/blob/6475398eb2a0e8fbef658ff3960f1f0c20741fb6/elastictransport/metrics.go#L43-L460)

We don't publish any of those at the moment through the docappender monitoring and have limited visibility on the performance and behavior of the client.

Describe the desired state

We would capture, transform to OTel format and publish the go-elasticsearch Transport metrics populated by the client lib.
With them we could be able to plot HTTP-request-level success rate (Failures / Requests ratio) as well as per-HTTP-status code rates (Responses by [key] / Requests).

Additional context

When designing how to expose the metrics, we should consider the possibility of adding arbitrary metadata to the data points exported. This is typically done with an attribute.Set in OTel, likely the existing MetricAttributes config field can be already serve this purpose.