Add BEYLA_CONFIG_PATH env var

[mariomac]

Adding @grafsean as a reviewer, as this PR affects documentation.

As an addition to the -config CLI argument, the introduced BEYLA_CONFIG_PATH env var would allow cleaner configuration in container-based environments, not forcing the users to override the /beyla command invocation (which should be kept as an internal detail).

The good-old -config CLI arg will keep working as usual.

[grcevski]

LGTM! I only have one suggestion/question, should we name this variable as BEYLA_CONFIG_PATH or BEYLA_CONFIG_FILE? I'm concerned that folks may think this is an environment variable containing the contents of the config, rather than path to the config file.

[mariomac]

@grcevski very good point. I'll change it to BEYLA_CONFIG_PATH

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (99088fd) 67.64% compared to head (2a3f712) 44.91%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #549       +/-   ##
===========================================
- Coverage   67.64%   44.91%   -22.73%     
===========================================
  Files          66       67        +1     
  Lines        5687     5628       -59     
===========================================
- Hits         3847     2528     -1319     
- Misses       1553     2946     +1393     
+ Partials      287      154      -133     

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unittests	44.91% <ø> (?)

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[grafsean]

Looks good, little copy touched!