feat(sdk): Add sampling multiplier config as an intermediate step to 100% sampling (#52104)

We want to eventually end up in a state where we sample at 100% client
(sdk) side.

Not being sure about the impact of just turning this on throughout the
system,
we will do it in steps with the new `SENTRY_MULTIPLIER_APM_SAMPLING`
config that we will bump in steps,
first on isolated instances and then everywhere till we either

* get to 100% sampling
* find and document problems that we can handle via [backpressure
management](getsentry/sentry-python#2189) in
SDKs

Author: sl0thentr0py

src/sentry/conf/server.py

@@ -1768,6 +1768,9 @@ def SOCIAL_AUTH_DEFAULT_USERNAME() -> str:
 # sample rate for all reprocessing tasks (except for the per-event ones)
 SENTRY_REPROCESSING_APM_SAMPLING = 0
 
+# upsampling multiplier that we'll increase in steps till we're at 100% throughout
+SENTRY_MULTIPLIER_APM_SAMPLING = 1
+
 # ----
 # end APM config
 # ----

src/sentry/utils/sdk.py

@@ -269,7 +269,14 @@ def traces_sampler(sampling_context):
             pass
 
     # Default to the sampling rate in settings
-    return float(settings.SENTRY_BACKEND_APM_SAMPLING or 0)
+    rate = float(settings.SENTRY_BACKEND_APM_SAMPLING or 0)
+
+    # multiply everything with the overall multiplier
+    # till we get to 100% client sampling throughout
+    if settings.SENTRY_MULTIPLIER_APM_SAMPLING:
+        rate = min(1, rate * settings.SENTRY_MULTIPLIER_APM_SAMPLING)
+
+    return rate
 
 
 def before_send_transaction(event, _):