Helm chart sets env vars the server never reads #92

The chart sets these env vars, but the code never reads them:

• PROMETHEUS_USERNAME / PROMETHEUS_PASSWORD: the password is a plain value, not a Secret, and no Prometheus tool sends auth.
• GRAFANA_URL / GRAFANA_API_KEY: there is no Grafana provider (see the gcx issue).
• OTEL_TRACING_ENABLED and OTEL_EXPORTER_OTLP_TRACES_TIMEOUT: the code reads OTEL_SDK_DISABLED, OTEL_SERVICE_NAME, etc. instead.
• KAGENT_NAMESPACE

Either wire them up (with Prometheus basic auth coming from a Secret) or remove them from the chart and README. Found while reviewing #91.