Be more precise about concurrency requirements

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/collector.go

@@ -40,7 +40,8 @@ type Collector interface {
 	// Collector may yield any Metric it sees fit in its Collect method.
 	//
 	// This method idempotently sends the same descriptors throughout the
-	// lifetime of the Collector.
+	// lifetime of the Collector. It may be called concurrently and
+	// therefore must be implemented in a concurrency safe way.
 	//
 	// If a Collector encounters an error while executing this method, it
 	// must send an invalid descriptor (created with NewInvalidDesc) to

prometheus/registry.go

@@ -107,9 +107,6 @@ type Registerer interface {
 	// Collector, and for providing a Collector that will not cause
 	// inconsistent metrics on collection. (This would lead to scrape
 	// errors.)
-	//
-	// It is in general not safe to register the same Collector multiple
-	// times concurrently.
 	Register(Collector) error
 	// MustRegister works like Register but registers any number of
 	// Collectors and panics upon the first registration that causes an