Move benchmarks outside of tests directory

[ocelotl]

Fixes #4029

[lzchen]

Can you explain with an example why we want to move these out of tests? The original issue does not provide much context.

[ocelotl]

Can you explain with an example why we want to move these out of tests? The original issue does not provide much context.

We do not want to run benchmarks in CI. Right now, to achieve that, we now have to install pytest-benchmark for every package that has benchmarks. For those packages that do not have benchmarks, we have to run pytest without --benchmark-skip. For those packages that have benchmarks, we need to run pytest with --benchmark-skip.

We can save ourselves all this trouble by just having the benchmarks in a different folder, so that we can just cleanly run pytest pointing to a folder without benchmarks in CI.

[ocelotl]

BTW, we already have moved benchmarks to another directory in contrib: open-telemetry/opentelemetry-python-contrib#2670