feat(telemetry): add discovered_gateways_count

[czeslavo]

What this PR does / why we need it:

Adds a discovered_gateways_count to a telemetry signal when the Gateway Discovery feature is turned on.

Which issue this PR fixes:

Solves #3779.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Patch coverage: 55.5% and project coverage change: +0.1 🎉

Comparison is base (98a3e4a) 56.7% compared to head (b5b3ac2) 56.8%.

❗ Current head b5b3ac2 differs from pull request most recent head 88dad8f. Consider uploading reports for the commit 88dad8f to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3783     +/-   ##
=======================================
+ Coverage   56.7%   56.8%   +0.1%     
=======================================
  Files        133     134      +1     
  Lines      15871   15904     +33     
=======================================
+ Hits        9014    9049     +35     
  Misses      6238    6238             
+ Partials     619     617      -2     

Impacted Files	Coverage Δ
internal/manager/telemetry/reports.go	0.0% <0.0%> (ø)
internal/manager/telemetry/manager.go	45.1% <37.5%> (+0.2%)	⬆️
...l/manager/telemetry/workflows/gateway_discovery.go	56.5% <56.5%> (ø)
internal/dataplane/clients_manager.go	96.8% <100.0%> (+0.1%)	⬆️

... and 2 files with indirect coverage changes

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