[BUG] filter added by update not present in monitoring in case of failure to start

[frikilax]

Describe the bug
When adding a filter to an already running Darwin session, if the filter fails to hot-start during the manager update command, it won't appear in monitoring.

Plateform (please complete the following information):

• OS (version): any
• Darwin version: 1.4

To Reproduce
Steps to reproduce the behavior:

1. setup a working set of filters to run
2. start Darwin and ensure every configured filter is running correctly
3. send a monitoring query to ensure every running filter has an entry in the monitoring reply
4. add any filter to darwin.conf, but with a wrong parameter (wrong path to configuration file, for example)
5. trigger a hot update using the manager command (see documentation for details)
6. ensure the manager couldn't start the new filter
7. end a second monitoring query
8. the new filter should not be in the existing entries

Expected behavior
when adding a new filter and hot-starting it, the filter's statistics should appear when querying the manager, wheter the filter was successfully started or not

Additional context
the filter's manager internal data is added only when the filter is successfully started