sampler: make sampler enabled by default

Enable sampler by default with 1% sample rate.

Release-Note-Core: Enabled Query Sampler by default with 1% sample
  rate.
Change-Id: I31387b29649d418ab78e59581bcb4f7d1ad23396
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/11025
Reviewed-by: Jason Brown <jason.b@readyset.io>
Tested-by: Buildkite CI

Author: altmannmarcelo

readyset/src/lib.rs

@@ -438,7 +438,7 @@ pub struct Options {
     pub tls_mode: TlsMode,
 
     /// Specifies the sample rate for the background query sampler. 0 means no sampling.
-    #[arg(long, env = "SAMPLER_SAMPLE_RATE", default_value = "0")]
+    #[arg(long, env = "SAMPLER_SAMPLE_RATE", default_value = "0.01")]
     sampler_sample_rate: f64,
 
     /// Specifies the maximum queue size for the background query sampler.