fix(datadog) correct valid sample_rate range

The sample_rate parameter for datadog represents sample rate
according to the statsd standard. As such, its valid range
is between 0 and 1.

See:
* https://docs.datadoghq.com/developers/faq/dog-statsd-sample-rate-parameter-explained/
* https://thenewstack.io/collecting-metrics-using-statsd-a-standard-for-real-time-monitoring/

kong/plugins/datadog/schema.lua

@@ -109,7 +109,7 @@ return {
                   { name = { type = "string", required = true, one_of = STAT_NAMES }, },
                   { stat_type = { type = "string", required = true, one_of = STAT_TYPES }, },
                   { tags = { type = "array", elements = { type = "string", match = "^.*[^:]$" }, }, },
-                  { sample_rate = { type = "number", between = { 1, math.huge }, }, },
+                  { sample_rate = { type = "number", between = { 0, 1 }, }, },
                   { consumer_identifier = { type = "string", one_of = CONSUMER_IDENTIFIERS }, },
                 },
                 entity_checks = {