rejigger metrics-observer so it's not part of tests

Cargo.toml

@@ -7,5 +7,5 @@ members = [
   "metrics-exporter-tcp",
   "metrics-exporter-prometheus",
   "metrics-tracing-context",
-  "metrics-observer",
 ]
+exclude = ["metrics-observer"]

metrics-tracing-context/tests/integration.rs

@@ -1,5 +1,3 @@
-use std::collections::HashSet;
-
 use metrics::{counter, KeyData, Label};
 use metrics_tracing_context::{LabelFilter, MetricsLayer, TracingContextLayer};
 use metrics_util::{layers::Layer, DebugValue, DebuggingRecorder, MetricKind, Snapshotter};