[CI] Benchmark, provide units with --benchmark_min_time

[marcalff]

Fixes #2620

Changes

Please provide a brief description of the changes here.

• Fix benchmark scripts to provide units to option --benchmark_min_time
• See related Allow specifying number of iterations via --benchmark_min_time. google/benchmark#1525
• Dump benchmark results in CI logs, to help troubleshooting.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[ThomsonTan]

Does this issue reach otel-cpp after the recent upgrade?

[marcalff]

Does this issue reach otel-cpp after the recent upgrade?

PR #2597 upgraded from benchmark 1.7.1 to 1.8.3, and the issue started at that point.

[marcalff]

Note that this PR is tentative:

• benchmark is not executed in CI for each PR, only in the main branch
• We will see after the merge to main if the CI is fixed or not, and if not, at least we will be able to inspect logs.