Add beyla_network_inter_zone_bytes metric

[mariomac]

In addition to the decoration of beyla flow bytes metric with the src_zone and dst_zone, Beyla now enables a new counter metric that only reports flow bytes for inter-zone traffic (when src_zone != dst_zone).

The reasons for adding a new metric:

• You don't have to add new attributes to the beyla_network_flow_bytes metric.
• PromQL queries remain simpler (no need to join beyla_network_flow_bytes for different values of src/dst zones)
• Can track inter-AZ traffic with different levels of granularity. You can keep inter-az traffic as simple as possible (only cluster and src/dst zones) or go to finest levels of granularity (e.g. source Pod) that would increase too much cardinality in the general-purpose beyla_network_flow_bytes` metric.

Documentation will go in a different PR.

[codecov]

Codecov Report

Attention: Patch coverage is 77.94118% with 30 lines in your changes missing coverage. Please review.

Project coverage is 67.08%. Comparing base (6c4ea88) to head (39321e8).

Files with missing lines	Patch %	Lines
pkg/export/prom/prom_net.go	55.00%	16 Missing and 2 partials ⚠️
pkg/export/otel/metrics_net.go	72.72%	9 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1600      +/-   ##
==========================================
- Coverage   71.01%   67.08%   -3.93%     
==========================================
  Files         197      197              
  Lines       19792    19871      +79     
==========================================
- Hits        14055    13331     -724     
- Misses       5058     5854     +796     
- Partials      679      686       +7     

Flag	Coverage Δ
integration-test	52.56% <52.94%> (-0.20%)	⬇️
k8s-integration-test	?
oats-test	34.67% <35.29%> (+0.04%)	⬆️
unittests	47.04% <77.94%> (+0.06%)	⬆️

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[grcevski]

LGTM! Very good idea!

[grcevski]

I think it's OK to merge without the Clang tidy check passing, we can resolve that later.