Fix monitoring when adding pools

[mdorier]

This PR fixes a bug that happens if we add a pool when monitoring (in particular time series) is enabled. The current problem is that because there is no time series structure allocated for the new pool, the default monitoring system will access an array out of range, causing a segfault.

To solve this problem, this PR makes the following changes:

• It adds monitoring callbacks invoked when pools and xstreams are added and removed, to give a chance for the monitoring framework (whether default or custom) to react to such a change (e.g. by allocating/deallocating the proper data structures);
• It implements these callbacks for the default monitoring framework, allocating and moving the the time series as appropriate when a pool is added/removed.

[codecov]

Codecov Report

Attention: Patch coverage is 50.38760% with 64 lines in your changes missing coverage. Please review.

Project coverage is 57.24%. Comparing base (7682c08) to head (54d3cf5).

Files	Patch %	Lines
src/margo-default-monitoring.c	0.00%	37 Missing ⚠️
src/margo-config.c	70.65%	22 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #274      +/-   ##
==========================================
- Coverage   57.41%   57.24%   -0.18%     
==========================================
  Files          69       69              
  Lines        9626     9735     +109     
  Branches     1252     1256       +4     
==========================================
+ Hits         5527     5573      +46     
- Misses       3332     3394      +62     
- Partials      767      768       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.