chore(ci): cleanup benchmarking scripts and add profiling capability

[brettlangdon]

Main change here is adding viztracer and a way to generate profiles from benchmark scenario runs.

List of changes:

• Adding PROFILE_BENCHMARKS=1 env var to trigger generating viztracer profiles for each scenario and putting the results in artifacts directory
• Support supplying . as a ddtrace version to install local version mounted to /src/ in the benchmark container
• Run the benchmark container with --network host to allow connecting to a local trace agent for the scenarios which rely on an agent (flask_simple, etc)
• Make sure latest version of pip is present (not that important, I just saw a version upgrade notice, so figured it doesn't hurt)
• Run build_docs when modifying benchmarks/README.rst since it is included in our docs

Checklist

• Change(s) are motivated and described in the PR description
• Testing strategy is described if automated tests are not included in the PR
• Risks are described (performance impact, potential for breakage, maintainability)
• Change is maintainable (easy to change, telemetry, documentation)
• Library release note guidelines are followed or label changelog/no-changelog is set
• Documentation is included (in-code, generated user docs, public corp docs)
• Backport labels are set (if applicable)
• If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

• Title is accurate
• All changes are related to the pull request's stated goal
• Description motivates each change
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Change is maintainable (easy to change, telemetry, documentation)
• Release note makes sense to a user of the library
• Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[brettlangdon]

Multi-process timeline view (zoomed out)

Individual process timeline (zoomed in)

Combined flamegraph across all processes

Custom SQL query

[brettlangdon]

Example docs changes: https://output.circle-artifacts.com/output/job/a4b94cf6-7a70-4f63-9a9e-96235573f737/artifacts/0/tmp/docs/benchmarks.html