rust: create multi-file run loader (#4343)

Summary:
This patch adds a `RunLoader` that reads from multiple event files
within a single run. For now, it only keeps track of scalars, and it
doesn’t do anything with the data that it reads. But it persists
everything that we’ll need for the rest of the pipeline.

Test Plan:
Unit tests included: they create a test log directory and write some
event files, then ensure that they can be read back properly.

wchargin-branch: rust-run-loader

Author: wchargin

tensorboard/data/server/BUILD

@@ -31,6 +31,7 @@ rust_library(
         "event_file.rs",
         "masked_crc.rs",
         "reservoir.rs",
+        "run.rs",
         "scripted_reader.rs",
         "tf_record.rs",
         "types.rs",
@@ -50,6 +51,9 @@ rust_library(
 rust_test(
     name = "rustboard_core_test",
     crate = ":rustboard_core",
+    deps = [
+        "//third_party/rust:tempfile",
+    ],
 )
 
 rust_doc_test(

tensorboard/data/server/data_compat.rs

@@ -18,7 +18,7 @@ limitations under the License.
 use crate::proto::tensorboard as pb;
 use pb::{summary::value::Value, summary_metadata::PluginData};
 
-const SCALARS_PLUGIN_NAME: &str = "scalars";
+pub(crate) const SCALARS_PLUGIN_NAME: &str = "scalars";
 
 /// Determines the metadata for a time series given its first event.
 ///

tensorboard/data/server/lib.rs

@@ -19,6 +19,7 @@ pub mod data_compat;
 pub mod event_file;
 pub mod masked_crc;
 pub mod reservoir;
+pub mod run;
 pub mod tf_record;
 pub mod types;