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

[mariomac]

Addresses #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

Attention: Patch coverage is 64.28571% with 15 lines in your changes missing coverage. Please review.

Project coverage is 67.35%. Comparing base (ea991e3) to head (5e5a359).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/internal/discover/attacher.go	43.47%	12 Missing and 1 partial ⚠️
pkg/export/prom/prom.go	0.00%	1 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (ea991e3) and HEAD (5e5a359). Click for more details.

HEAD has 2 uploads less than BASE

Flag	BASE (ea991e3)	HEAD (5e5a359)
k8s-integration-test	1	0
integration-test	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1225       +/-   ##
===========================================
- Coverage   81.72%   67.35%   -14.37%     
===========================================
  Files         137      137               
  Lines       11483    11508       +25     
===========================================
- Hits         9384     7751     -1633     
- Misses       1560     3234     +1674     
+ Partials      539      523       -16     

Flag	Coverage Δ
integration-test	?
k8s-integration-test	?
oats-test	36.15% <64.28%> (+0.06%)	⬆️
unittests	52.88% <4.76%> (-0.17%)	⬇️

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