Add benchmarks to every type of package in build workflow

[NathanielRN]

In #1443 we add performance tests for the SDK.

The SDK falls under the category core of tests matched in the build workflow. For this reason, when the jq command looks through the entire repo after the workflow for *-benchmark.json files we are guaranteed to find at least ` file. This file is then given to the subsequent Github action to report on benchmarking results.

However, the build workflow also has instrumentation and exporter packages. These don't have performance tests yet so the jq command will fail. Even if we suppress the failure, the subsequent GitHub action will fail if no valid output.json file is created.

A temporary solution was to only run those 2 steps of the workflow if: matrix.package == 'core'. A more long-term solution will be to add benchmarks to instrumentation and exporter.

We know only these packages are required because as long as subsequent tests fit under these categories we won't have to keep adding if statements.

[NathanielRN]

Related Contrib Issue: open-telemetry/opentelemetry-python-contrib#249

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