Replace SendEncodeMetric with SendSyncEncodeMetric

This will allow the default `Registry` to be used in multi-threaded
environments (e.g. web servers) where the registry needs to be shared
between threads.

Closes prometheus#57.

src/encoding/text.rs

@@ -404,11 +404,11 @@ impl EncodeMetric for Box<dyn EncodeMetric> {
     }
 }
 
-pub trait SendEncodeMetric: EncodeMetric + Send {}
+pub trait SendSyncEncodeMetric: EncodeMetric + Send + Sync {}
 
-impl<T: EncodeMetric + Send> SendEncodeMetric for T {}
+impl<T: EncodeMetric + Send + Sync> SendSyncEncodeMetric for T {}
 
-impl EncodeMetric for Box<dyn SendEncodeMetric> {
+impl EncodeMetric for Box<dyn SendSyncEncodeMetric> {
     fn encode(&self, encoder: Encoder) -> Result<(), std::io::Error> {
         self.deref().encode(encoder)
     }

src/registry.rs

@@ -58,7 +58,7 @@ use std::ops::Add;
 /// #                "# EOF\n";
 /// # assert_eq!(expected, String::from_utf8(buffer).unwrap());
 /// ```
-pub struct Registry<M = Box<dyn crate::encoding::text::SendEncodeMetric>> {
+pub struct Registry<M = Box<dyn crate::encoding::text::SendSyncEncodeMetric>> {
     prefix: Option<Prefix>,
     labels: Vec<(Cow<'static, str>, Cow<'static, str>)>,
     metrics: Vec<(Descriptor, M)>,