Add documentation example for processor package (#5675)

[heitorganzeli]

Description

This PR introduces a testable example to the package processor. it's a similar example to the documentation one on https://opentelemetry.io/docs/collector/building/receiver/#designing-and-validating-receiver-settings

Issue: #5675 (comment)

Testing

the testable example was executed successfully:

Documentation

this testable example follows https://go.dev/blog/examples#larger-examples format and should be automatically added to processor package documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.61%. Comparing base (dc3f5d2) to head (8e65c5f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13070   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files         505      505           
  Lines       28409    28409           
=======================================
  Hits        26027    26027           
  Misses       1873     1873           
  Partials      509      509           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[mx-psi]

Based on the go.mod changes, I wonder if it's better to have this as a processorhelper example instead?

[heitorganzeli]

i can move it there.

but, as a guideline, what do you think would be the default way to build processors?

as a new guy, processorhelper seemed like a more streamline way of doing it.

maybe processor should have two examples, one without and one with it (or a link to the one on processorhelper)