[8.x](backport #4037) Support sampling rate in APM configuration

What is the problem this PR solves?

This PR introduces support for APM sample rate using the values provided by elastic-agent though the control protocol // Please do not just reference an issue. Explain WHAT the problem this PR solves here.

How does this PR solve the problem?

Added a check for a value of transacton sampling in apmConfigToInstrumentation (it's an optional float32) and formatted the vale as a string to be included in the instrumentation copnfig. // Explain HOW you solved the problem in your code. It is possible that during PR reviews this changes and then this section should be updated.

How to test this PR locally

Design Checklist

[ ] I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
[ ] I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
[ ] I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[ ] I have made corresponding change to the default configuration files
[ ] I have added tests that prove my fix is effective or that my feature works
[ ] I have added an entry in ./changelog/fragments using the changelog tool

Related issues

This is an automatic backport of pull request #4037 done by Mergify.

elastic / fleet-server