docs: add durable example

[karel-rehor]

Proposed Changes

Adds an example showing how to reuse and keep alive client instances for the duration of an application, as recommended in GRPC Best Practices. Clients should be created or recreated as little as possible and should be reused as much as possible.

The example illustrates using a primitive client pool that manages client instances while running four threads simultaneously handling influxdb calls. Two of these threads produce error responses from the server. Clients recover from these errors and can still be used for other calls.

This was motivated by research into a customer question about using connection pools for the query transport. Connection pools are already handled by the underlying GRPC ManagedChannel, which is designed to be long-running and robust. The write transport is similarly designed to recover from typical error responses.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate - not needed
• Tests pass
• Commit messages are conventional
• Sign CLA (if not already signed)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.14%. Comparing base (845dfb2) to head (010fa6a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #253   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files          20       20           
  Lines        1207     1207           
  Branches      188      188           
=======================================
  Hits         1088     1088           
  Misses         42       42           
  Partials       77       77           

☔ 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.

[jansimonb]

Nice work, just few comments.

[karel-rehor]

@jansimonb

I've made some recommended changes. I've also expanded explanations in comments. Since I ran across a flaky test failure in QueryOptonsTest.java following a couple of commits, I decided to make a small change to clear up some of the flake. It seems when tests run in parallel a fixed port value has been causing an occasional BindException.

[jansimonb]

LGTM, thanks! 👍