Allow customize stacktrace

[JoniJnm]

The elastic guide says:

Elastic APM agents collect stacktrace information under certain circumstances. This can be very helpful in identifying issues in your code, but it also comes with an overhead at collection time and increases the storage usage.

Stack trace collection settings are managed in each agent.

But PHP APM agent doesn't allow to customize the stacktrace as other agent do

• https://www.elastic.co/guide/en/apm/agent/php/current/configuration.html
• https://www.elastic.co/guide/en/apm/agent/java/current/config-stacktrace.html#config-stack-trace-limit

At least stack_trace_limit would be nice