fix/add datadog tracing exporter

[GuillaumeDecMeetsMore]

Changed

• Add (unofficial) Datadog trace exporter
• Add simple logger middleware for request
• Add #[derive(debug)] on repositories' structs
  • This is required (for printing the "state" of the repository inside the span)
• Add #[instrument] on all repositories' methods
  • This simply wraps the processing inside a span (APM)
• Adapt Tracing configuration so that
  • It uses Datadog exporter if DATADOG_TRACING_ENDPOINT env var is defined
  • It uses OpenTelemetry HTTP exporter if OTLP_TRACING_ENDPOINT env var is defined
• Adapt logger so that it prints JSON (but not Bunyan format anymore)

Todo (later - in other PRs)

• Add log processing pipeline in DD
• Adapt as much as possible logs' format to match DD expected format (without making it too complex)