feature: Added gRPC close signal to Monitor call (allows graceful close of monitor)

[cmaglie]

Please check if the PR fulfills these requirements

See how to contribute

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Adds a close feature to the gRPC Monitor call. This new feature allows the client to gracefully close the port monitor.

What is the current behavior?

To close a gRPC-opened monitor, the client must close the gRPC streaming call, BTW this creates a race condition: the client is not aware of when the connection to the port is actually closed by the daemon afterward.
See #2271 for details.

What is the new behavior?

The close message should allow to gracefully close the monitor.

Does this PR introduce a breaking change, and is titled accordingly?

Yes, the gRPC Monitor call has been refactored to use a better description of the gRPC message based on the oneof descriptor.

Other information

Fix #2271

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (07cf265) 68.71% compared to head (094e3a5) 68.85%.

Files	Patch %	Lines
commands/daemon/daemon.go	69.56%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2276      +/-   ##
==========================================
+ Coverage   68.71%   68.85%   +0.13%     
==========================================
  Files         204      204              
  Lines       20446    20465      +19     
==========================================
+ Hits        14050    14091      +41     
+ Misses       5251     5220      -31     
- Partials     1145     1154       +9     

Flag	Coverage Δ
unit	68.85% <73.07%> (+0.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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