feat(php_fpm): track maximum active processes

The PHP-FPM status page also provides the "max active processes" statistic, which translates to the maximum number
of processes that can be executed at once. In the PHP-FPM configuration, this is controlled by the pm.max_children
key. This is particularly useful in understanding saturation within the PHP-FPM container, in a manner that allows
horizontal scaling in a more intelligent way than the built-in CPU and memory metrics in Kubernetes. This pairs
well with the Datadog External Metrics functionality as well

php_fpm/datadog_checks/php_fpm/php_fpm.py

@@ -59,6 +59,7 @@ class PHPFPMCheck(AgentCheck):
         'idle processes': 'php_fpm.processes.idle',
         'active processes': 'php_fpm.processes.active',
         'total processes': 'php_fpm.processes.total',
+        'max active processes': 'php_fpm.processes.max',
     }
 
     MONOTONIC_COUNTS = {