fix(kafka source): fix acknowledgement handling during shutdown and r…

…ebalance events (vectordotdev#17497)

* test(kafka source): integration tests for acknowledgement handling during shutdown and rebalance events

When running locally, this is most easily tested using an existing kafka
topic pre-populated with, say 100k messages:

❯ kcat -b $BROKER_ADDR -P -t TestTopic -K : -l <(for i in $(seq 1 100000); do echo "${i}:{\"value\": ${i}}"; done);

...then running the tests, targeting that topic through environment variables. This can be a bit finicky with regard to timings,
so KAFKA_SHUTDOWN_DELAY controls how long to run the first consumer before shutting down (drain at shutdown test), and
KAFKA_CONSUMER_DELAY controls the time between starting new consumers during the rebalancing test.

❯ KAFKA_SEND_COUNT=0 \
  KAFKA_EXPECT_COUNT=100000 \
  KAFKA_TEST_TOPIC=TestTopic \
  KAFKA_CONSUMER_DELAY=5000 \
  KAFKA_SHUTDOWN_DELAY=5000 \
  KAFKA_HOST=$BROKER_ADDR \
  KAFKA_PORT=9092 \
    cargo test --lib --no-default-features -F sources-kafka -F kafka-integration-tests drains_acknowledgement

* fix(kafka source): drain pending acknowledgements on shutdown and rebalance

* fix(kafka source): performance improvements for acknowledgement handling on many partitions

Instead of tokio StreamMap, which gets very slow when more than a handful of partitions are involved,
use a task and forwarding channel for each partition. Introduces a
little bookkeeping, but scales well to at least hundreds of
partitions

* clippy fixes, and remove unnecessary last_offset tracking

* cargo fmt again

* fmt

* clean up handle_messages loop and add a tracing span for metrics collection

* fixup changes lost after merging master

* clippy warning

* enhancement(kafka source): kafka source uses a dedicated task per partition to consume & acknowledge messages

* make the spelling checker happy, maybe?

* emit a debug log instead of panicking if a shutdown happens during a consumer rebalance

* improved partition eof handling

* add OptionFuture to drain deadline and EOF handling, and use is_subset to detect when all expected partitions are finished draining

* replace OnceCell with OnceLock

* cargo fmt

* create clear distinction between consuming and draining states

* add "complete" as a terminal state, and "keep_consuming", "keep_draining", and "complete" methods for descriptive state (non)transitions

* use state transition methods consistently for all state transitions

* slightly clearer assertion messages about what is expected

* update obsolete comment, make coordinator loop condition explicit

* use keep_consuming from the drain_timeout while consuming handler

* rely solely on adding/removing entries in expect_drain to detect when draining is complete

* fix comment :P

* clippy/fmt fixes

* minor cleanup: during shutdown, use is_drain_complete to detect the already-finished state

* integration test uses `FuturesUnordered` for better performance

Co-authored-by: Doug Smith <dsmith3197@users.noreply.github.com>

* use FuturesUnordered

* use 6 partitions for integration test

* integration test using 125k messages

* add drain_timeout_ms option for kafka source

* enforce drain_timeout_ms < session_timeout_ms when building kafka source

* generate component docs

* use Option::{unzip, map_or} methods

Co-authored-by: Bruce Guenter <bruce@untroubled.org>

* remove OnceLock on callback channel sender, and other review cleanups

- avoid panic in case split_partition_queue returns None
- remove `pub` markers that are not needed
- add comments around drain coordination signalling
- cargo fmt

* members of Keys struct are cloned once per consumed partition, instead of entire config object

* cargo fmt and fix clippy warnings

---------

Co-authored-by: Doug Smith <dsmith3197@users.noreply.github.com>
Co-authored-by: Bruce Guenter <bruce@untroubled.org>