chore(antithesis): Add provider_kind in sample config

Pyld54 asserts a v3 tagset back-reference resolves to an earlier
tagset. Agent emits that back-reference only when a metric carries both
a shared prefix tagset and its own tags. Previously we only ever emitted
load that carried no shared prefix.

This commit adds provider_kind into the sampled datadog.yaml. Agent will
attach this as a global tag which acts as a shared prefix. 1/3 of
configs will not have provider_kind present.

Author: blt

test/antithesis/harness/src/config.rs

@@ -117,6 +117,9 @@ impl DogStatsdConfig {
 /// unset. The current value caps the preallocation at 512 MiB.
 const MAX_STRING_INTERNER_ENTRIES: u64 = 1_048_576;
 
+/// Cloud provider kinds sampled into `provider_kind`. The empty string emits no tag.
+const PROVIDER_KINDS: &[&str] = &["", "gke-autopilot", "gke-gdc"];
+
 /// Agent-facing config. `hostname`, `api_key`, `dd_url`, and the socket are
 /// supplied by the environment; `log_level` and the `DogStatsD` options are
 /// sampled per branch. The static flags are appended by [`Self::to_yaml`], not
@@ -132,6 +135,9 @@ pub struct DatadogConfig {
     dd_url: String,
     /// Agent log verbosity. Pinned to `error` (see [`LogLevel`]).
     log_level: LogLevel,
+    /// Cloud provider kind, sampled per timeline.
+    #[serde(skip_serializing_if = "String::is_empty")]
+    provider_kind: String,
     /// `DogStatsD` options, flattened to top-level `dogstatsd_*` keys.
     #[serde(flatten)]
     dogstatsd: DogStatsdConfig,
@@ -150,6 +156,7 @@ impl DatadogConfig {
             api_key: api_key.to_owned(),
             dd_url: dd_url.to_owned(),
             log_level: LogLevel::Error,
+            provider_kind: PROVIDER_KINDS[rng.random_range(0..PROVIDER_KINDS.len())].to_owned(),
             dogstatsd: DogStatsdConfig::sample(rng, dogstatsd_socket),
         }
     }