Fixes process metrics when the application has not sent/received any request

[mariomac]

Addresses https://github.com/grafana/beyla/issues/1219 by sending a "Process Alive" signal that would reach the process metrics pipeline even before any other client/server request are instrumented.

Use cases:

• Provides process metrics during long initialization times
• Provides process metrics in applications with unsupported protocols (but you can still see them via network metrics)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.76%. Comparing base (ea991e3) to head (5e5a359). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1225      +/-   ##
==========================================
+ Coverage   81.72%   81.76%   +0.03%     
==========================================
  Files         137      137              
  Lines       11483    11508      +25     
==========================================
+ Hits         9384     9409      +25     
  Misses       1560     1560              
  Partials      539      539              

Flag	Coverage Δ
integration-test	60.85% <100.00%> (+0.10%)	:arrow_up:
k8s-integration-test	58.46% <97.61%> (+0.16%)	:arrow_up:
oats-test	36.15% <64.28%> (+0.06%)	:arrow_up:
unittests	52.88% <4.76%> (-0.17%)	:arrow_down:

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