DOCS: Add more detail to Tempo troubleshooting section

[mellieA]

Provide more detail in this section: https://grafana.com/docs/tempo/latest/troubleshooting/agent/#metrics to describe where the metrics can be viewed.

The agent publishes a few Prometheus metrics that are useful to determine how much trace traffic it is receiving and successfully forwarding. These are a good place to start when diagnosing tracing Agent issues.

traces_receiver_accepted_spans traces_receiver_refused_spans traces_exporter_sent_spans traces_exporter_send_failed_spans

Suggest adding:

The metrics are visible at http://localhost:12345/metrics if you are using the Grafana Agent to send traces to Grafana Cloud. In your Grafana Cloud instance, they can be checked in the grafanacloud-usage datasource:

[GrafanaWriter]

@KMiller-Grafana - FYI. Triaging to backlog.

[knylander-grafana]

These updates could go https://grafana.com/docs/tempo/latest/grafana-agent/

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