contrib/google.golang.org/grpc: isolate tests using an independent rig instance instead of a shared one

[darccio]

What does this PR do?

This PR avoids flaky tests in contrib/google.golang.org/grpc that were using a shared rig instance concurrently.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-10-23 08:58:11

Comparing candidate commit f495b11 in PR branch dario.castane/port-fix-appsec-tests with baseline commit 00055c7 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 39 metrics, 2 unstable metrics.

[darccio]

@Julio-Guerra Can you review this change? It was released as part of v1.56.0 but never merged into main.