fix(otel): restore thanos discovery and add traffic generator

[victoriacheng15]

Summary

Fixed Thanos discovery pipeline and established end-to-end telemetry verification. This update resolves gRPC connectivity between Thanos Query and the Prometheus sidecar and introduces a synthetic traffic generator for pipeline validation.

List of Changes

• Fix (tofu): Renamed extraArgs to extraFlags in Thanos Helm release to correctly pass the --endpoint argument to the query container, resolving the empty metrics issue.
• Feat (tofu): Added a managed kubernetes_service_v1 for prometheus-thanos-grpc to ensure reliable headless discovery of the Prometheus sidecar, bypassing Helm chart limitations.
• Fix (k3s): Disabled redundant dnsDiscovery in Thanos Query values to eliminate SRV lookup errors and prioritize the direct gRPC endpoint connection.
• Feat (scripts): Created traffic_generator.sh to produce burst and continuous synthetic traffic, enabling empirical verification of the OTLP-to-Thanos telemetry pipeline.

Verification

• tofu apply successfully provisioned the prometheus-thanos-grpc service.
• thanos-query logs confirmed successful sidecar discovery: "adding new sidecar...".
• Executed ./scripts/traffic_generator.sh --burst and verified 20/20 metrics appeared in Thanos via proxy_synthetic_request_total.