Indicate where artifacts are going to and coming from in the log

Description

Indicate where artifacts are coming from and being pushed to in the logs.

Changes proposed in this merge request:

• Make Element.get_brief_display_key() public for logging purposes of other modules
• Add info messages to indicate the remote being used for pushing to a cache
• Add info messages to indicate the remote being used for pulling from a cache

This merge request, when approved, will close issue/bug: #275 (closed)

CHANGELOG/Release Notes

• Element.get_brief_display_key() is now publicly accessible.

Preview

[--:--:--][72f8f716][ pull:hello.bst                     ] START   CAS-interaction/hello/72f8f716-pull.511.log
[--:--:--][72f8f716][ pull:hello.bst                     ] INFO    Pulling 72f8f716 <- remote (https://localhost:9999)
[--:--:--][72f8f716][ pull:hello.bst                     ] INFO    Downloaded artifact 72f8f716
[00:00:00][72f8f716][ pull:hello.bst                     ] SUCCESS CAS-interaction/hello/72f8f716-pull.511.log

...

[--:--:--][72f8f716][ push:hello.bst                     ] START   CAS-interaction/hello/72f8f716-push.1010.log
[--:--:--][72f8f716][ push:hello.bst                     ] START   Pushing artifact 72f8f716
[--:--:--][72f8f716][ push:hello.bst                     ] INFO    Pushing 72f8f716 -> https://localhost:9999
[--:--:--][72f8f716][ push:hello.bst                     ] INFO    Pushed artifact 72f8f716
[00:00:00][72f8f716][ push:hello.bst                     ] SUCCESS Pushing artifact 72f8f716
[00:00:00][72f8f716][ push:hello.bst                     ] SUCCESS CAS-interaction/hello/72f8f716-push.1010.log

buildstream/_artifactcache/cascache.py

@@ -220,6 +220,8 @@ class CASCache(ArtifactCache):
         for remote in self._remotes[project]:
             try:
                 remote.init()
+                
+                element.info("Pulling {} <- {}".format(element._get_brief_display_key(), remote.spec.url))
 
                 request = buildstream_pb2.GetReferenceRequest()
                 request.key = ref
@@ -228,7 +230,6 @@ class CASCache(ArtifactCache):
                 tree = remote_execution_pb2.Digest()
                 tree.hash = response.digest.hash
                 tree.size_bytes = response.digest.size_bytes
-
                 self._fetch_directory(remote, tree)
 
                 self.set_ref(ref, tree)
@@ -238,6 +239,7 @@ class CASCache(ArtifactCache):
 
             except grpc.RpcError as e:
                 if e.code() != grpc.StatusCode.NOT_FOUND:
+                    element.info("{} not found at remote {}".format(element._get_brief_display_key(), remote.spec.url))
                     raise
 
         return False
@@ -258,7 +260,7 @@ class CASCache(ArtifactCache):
         push_remotes = [r for r in self._remotes[project] if r.spec.push]
 
         pushed = False
-        
+
         for remote in push_remotes:
             remote.init()