[exporter/exporterhelper] Add exporter retry-drop metrics

[iblancasa]

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Component(s)

exporter/exporterhelper

Is your feature request related to a problem? Please describe.

We currently record otelcol_exporter_send_failed_* counters inside the exporter helper whenever a send call fails, regardless of whether the retry pipeline will ultimately drop the data. This makes it hard to distinguish transient failures that were retried successfully from failures that depleted all retry attempts. We should surface explicit telemetry and an error marker so operators can tell when data was permanently dropped because retries were exhausted.

Alerting on the existing *_send_failed_* counters fires on every transient retry, leading to noisy signals and unclear remediation paths.

Operators need to know when the retry policy ran out of attempts so they can react (e.g., scale the exporter or adjust retry settings) instead of chasing transient blips.

Exporter authors and tests should have an ergonomic way to detect when the retry sender gives up, rather than parsing error strings.

Describe the solution you'd like

Create a new metric.

Describe alternatives you've considered

No response

Additional context

No response

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