Observability docs have no Elastic guidance for tracing or metrics

Within `daprdocs/content/en/operations/observability`, Elastic appears only once: the FluentD logging how-to. There is no Elastic page under `tracing/` or `metrics/`, while comparable backends are covered in each section.

| Section | Elastic | New Relic, for comparison |
| --- | --- | --- |
| `tracing/` | none | yes |
| `metrics/` | none | yes |
| `logging/` | FluentD how-to | yes |

Elastic ships an OpenTelemetry Collector distribution (EDOT) built into the Elastic Agent, which reads a standard collector configuration. A single collector can carry Dapr traces, metrics and logs, so the guidance fits naturally into the existing per-signal structure.

Start in daprdocs/content/en/operations/observability and inspect the existing tracing/ and metrics/ pages, using the New Relic guidance as a structural comparison. Also review the existing Elastic-related FluentD logging how-to and the repository's documentation conventions. Done means Elastic guidance exists under both tracing/ and metrics/ and explains how the standard collector configuration carries Dapr signals.

Written by the indexing model from the issue text.