[DEV-15058] Add signoz, disabled by default

[ltellesfl]

Note

Medium Risk
Introduces a new monitoring mode that conditionally disables existing Prometheus/Loki/Tempo/alerting components in favor of SigNoz, which could change observability coverage if misconfigured. Default is off, limiting impact unless enable_signoz is enabled and endpoints/tokens are set.

Overview
Adds an optional SigNoz integration (disabled by default) via new Terraform variables (enable_signoz, signoz_otel_collector_endpoint, signoz_bearer_token) and new Helm values for signoz plus enabling the opentelemetry-operator.

When enable_signoz is true, the PR turns off the existing monitoring stack pieces (kube-prometheus-stack, opentelemetry-collector, tempo, loki/fluent-bit, and Alertmanager-based alerting) to avoid running both stacks simultaneously.

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