docs(correctness): describe OTLP host fixture payload

Document the static OTLP metrics payload used by the host permutation correctness case so reviewers can see which host inputs the base64 protobuf covers.

Author: thieman

test/correctness/cases/otlp-metrics-host/millstone.yaml

@@ -5,4 +5,6 @@ volume: 1000
 corpus:
   size: 1
   payload:
+    # ExportMetricsServiceRequest with four otlphost.metric gauge points that differ only by resource host:
+    # unset host, host.name="", host.name="correctness-testing", and host.name="custom".
     static_base64: "Ci0KABIpEicKD290bHBob3N0Lm1ldHJpYyoUChIZAMqaOwAAAAAhAAAAAAAA8D8KPgoRCg8KCWhvc3QubmFtZRICCgASKRInCg9vdGxwaG9zdC5tZXRyaWMqFAoSGQDKmjsAAAAAIQAAAAAAAABAClEKJAoiCglob3N0Lm5hbWUSFQoTY29ycmVjdG5lc3MtdGVzdGluZxIpEicKD290bHBob3N0Lm1ldHJpYyoUChIZAMqaOwAAAAAhAAAAAAAACEAKRAoXChUKCWhvc3QubmFtZRIICgZjdXN0b20SKRInCg9vdGxwaG9zdC5tZXRyaWMqFAoSGQDKmjsAAAAAIQAAAAAAABBA"