Introduce a ScannerStartupDelay configuration to enable WAL write-only
recovery, allowing fence messages to be persisted during
primary–secondary switchover when the StreamingNode is trapped in crash
loops.
issue: https://github.com/milvus-io/milvus/issues/46368
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **New Features**
* Added a configurable WAL scanner pause/resume and a consumer request
flag to optionally ignore pause signals.
* **Metrics**
* Added a scanner pause gauge and pause-duration tracking for WAL
scanning.
* **Tests**
* Added coverage for pause-consumption behavior and cleanup in stream
client tests.
* **Chores**
* Consolidated flush-all logging into a single field and added a helper
for bulk message conversion.
<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
---------
Signed-off-by: bigsheeper <yihao.dai@zilliz.com>
issue: #38399
- add metrics for broadcaster component.
- add metrics for wal flusher component.
- add metrics for wal interceptors.
- add slow log for wal.
- add more label for some wal metrics. (local or remote/catcup or
tailing...)
Signed-off-by: chyezh <chyezh@outlook.com>
