fix: batch pip installs in bootstrap for faster installs

[Manvi2402]

Description

Previously, _run_install in bootstrap.py called pip install once per
package in a loop, causing multiple subprocess spawns, pip loads, and
dependency resolution passes. For a typical project with ~8 instrumentors,
this took 60-120 seconds.

This change batches all packages into a single pip install call, reducing
it to 15-25 seconds (4-6x speedup).

Fixes #4484

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Ran pre-commit checks — all passed
• Verified syntax with python -m py_compile

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[herin049]

This needs to be updated to link to the PR

[Manvi2402]

Updated the changelog to link to the PR instead of the issue. Thanks for the review.

[idan-rahamim-lendbuzz]

Thanks For your contribution !
Can you also add to the opentelemetry-bootstrap -a install command a flag support to disable the -U of pip install?
In a fresh environment (e.g., Docker build), there's nothing to upgrade, so the upgrade check is pure overhead

[Manvi2402]

Thanks for the suggestion. Added a --no-upgrade flag to disable the -U option for fresh environments like Docker builds.