Document the proper way to send traces, metrics and logs to Datadog

[Geal]

Telemetry with Datadog has been challenging, due to the differences in behaviour with Opentelemetry, and the limitations of the opentelemetry-datadog crate.
For some context:

• opentelemetry-datadog can send spans but not metrics because the metrics exporter is not implemented yet
• we can use opentelemetry-otlp to send data to the datadog agent, which will handle spans and metrics with configuration like DD_OTLP_CONFIG_RECEIVER_PROTOCOLS_GRPC_ENDPOINT=0.0.0.0:4317 (it needs fix histogram metrics in OTLP #2564 though)
• the datadog agent is unable to extracts the logs that are sent as opentelemetry events Errors in traces merely showing error=true #2402
• it looks like log collection should instead be done separately, either from files, journald or others https://docs.datadoghq.com/agent/logs/?tab=tailfiles#custom-log-collection
• but it's been pointed to me that the trace id and span id must be updated to allow Datadog to correlate them with traces: https://github.com/onlydustxyz/marketplace/blob/7decda6f25f63b876b757cd4d4337a1f364441f5/backend/common/olog/src/lib.rs

We should document a proper way to set up telemetry with datadog, so users do not spend too much time tweaking and testing

[o0Ignition0o]

We will have an other pass at this for #3226