Add Kafka topic template routing for Kubernetes pod log streams

[jeremyudis]

Summary

Adds writer.kafka.topicTemplate, letting a single log config running in Kubernetes mode (DaemonSet tailing /var/log/pods) route each pod log stream to a Kafka topic derived from its source.

Supported variables:

• %{namespace} — the pod's Kubernetes namespace (parsed from the namespace_podname_uid pod directory name)
• %{container} — the container directory in the standard kubelet layout /var/log/pods/<ns>_<pod>_<uid>/<container>/0.log (use logDir=/* so each container directory becomes its own stream)
• %{metadata:<key>} — a pod label/annotation fetched via the existing podMetadataFields mechanism, enabling self-serve routing (teams annotate their pods with a topic name)

Topic resolution precedence (per stream, at writer creation)

1. topicTemplate — used only when every variable resolves and the result is a legal Kafka topic name
2. topic — the required static fallback; legacy \N capture-group expansion against logStreamRegex is unchanged. Fallbacks emit singer.writer.topic_template_fallback

Design notes

• Pod-level identifiers (%{podName}, %{podUid}) are deliberately rejected at config load time: pods churn on every restart/reschedule, so they would create unbounded topic cardinality.
• Validation is fail-fast: unsupported variables, malformed %{...} syntax, and illegal topic characters are all ConfigurationExceptions at startup.
• One Kafka producer per cluster is already shared (KafkaProducerManager), so many resolved topics add no producer overhead.
• Also fixes RecursiveFSEventProcessor to collect configs from all matching logConfigMap entries instead of stopping at the first, so a wildcard logDir config (e.g. /*) no longer permanently shadows an exact-directory config at the same depth.

Docs

• docs/configuration_samples/sample_kubernetes/README.md — Kubernetes mode, container stdout tailing, routing precedence, DaemonSet deployment notes
• docs/configuration_samples/sample_kubernetes/conf.d/container.stdout_logs.properties — working per-container routing example

Test plan

• TestTopicTemplateResolver (17 tests): template validation (rejects pod-level/unknown variables, malformed syntax, illegal characters), namespace/container/metadata resolution, all unresolvable-variable and illegal-topic fallback paths
• TestTopicTemplateRouting (5 tests): writer-factory precedence — template wins when resolvable, falls back to static topic otherwise, legacy capture-group expansion preserved
• TestContainerLogStreams (3 tests): end-to-end with the kubelet fixture — logDir=/* produces one stream per container directory with concrete paths resolving to logs_default_web / logs_default_sidecar; wildcard and exact configs no longer shadow each other
• TestLogConfigUtils (+4 tests): config parsing accepts valid templates and rejects invalid ones at load time
• Full regression over the touched areas: TestPodLogCycle, TestPodAllowlist, TestKubeService, TestRecursiveEventProcessor — 71/71 passing

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