test: switch OTLP default transport to gRPC in examples

[codefromthecrypt]

Description

Switches OTLP default transport from HTTP to gRPC in test fixtures and examples.

This prepares for Envoy Gateway integration which currently only supports OTLP/gRPC for access logs. Once Envoy Gateway exposes OTLP/HTTP (pending upstream changes in Envoy), we can switch back.

Related Issues/PRs (if applicable)

• access_log: add OTLP/HTTP exporter to OpenTelemetry access logger envoy#42445 (OTLP/HTTP access logs in Envoy)
• feat(telemetry): add custom headers for OTLP exports (metrics, tracing, access logs) gateway#7674 (OTLP headers support)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.26%. Comparing base (2b2d4a5) to head (14d5093).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1774   +/-   ##
=======================================
  Coverage   84.26%   84.26%           
=======================================
  Files         117      117           
  Lines       12911    12911           
=======================================
  Hits        10879    10879           
  Misses       1384     1384           
  Partials      648      648           

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[nacx]

Thx!