feat: Add Throughput and Lag Metrics Per Topic - #21
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TopicMetrics records produced and consumed against a topic. Lag is log end offset minus the consumer's committed offset. Rates use a sliding window of circular time buckets. Restores DeadLetterQueue barrel exports dropped by the work-queue merge.
ThomasHartDev
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Operators live on two numbers: how far a consumer is behind the log, and how fast the topic is moving. This adds TopicMetrics for that.
produced appends a 0-based offset. consumed advances that consumer's commit. Lag is logEndOffset minus committedOffset, so billing and search on the same topic can report different lag and different oldest-message age. Produce and consume rates are a sliding window of circular time buckets. A burst that then goes quiet reports 0 without wiping the lifetime offsets.
Puts the DeadLetterQueue re-exports back on the package barrel. The work-queue merge dropped them and typecheck has been failing the existing DLQ tests.
Closes #20.