Backend guides

OxiPulse sends OTLP/gRPC to any OpenTelemetry backend. Each guide is tested end to end in CI on every change, except SigNoz for now.

• SigNozDirectly over OTLP/gRPC, self-hosted or SigNoz Cloud. Not yet tested in CI.
• OpenObserveDirectly over OTLP/gRPC with Basic auth headers.
• UptraceDirectly over OTLP/gRPC with the project DSN header.
• VictoriaMetricsThrough an OTel Collector, with Prometheus-style names.
• Prometheus + GrafanaThrough an OTel Collector, with a ready-made dashboard.