Buffer JSON slog messages

serde_json will happily do a `write` syscall for every token it
serializes. Add a small 100-character BufWriter buffer to prevent
excessive syscalls.

Note that serde_json does not appear to call `Write::flush` so short
lines will not be printed until the buffer is full.

foundations/src/telemetry/log/init.rs

@@ -20,6 +20,7 @@ use slog_json::{Json as JsonDrain, Json};
 use slog_term::{FullFormat as TextDrain, PlainDecorator, TermDecorator};
 use std::fs::File;
 use std::io;
+use std::io::BufWriter;
 use std::panic::RefUnwindSafe;
 use std::sync::Arc;
 
@@ -70,6 +71,9 @@ pub(crate) fn init(service_info: &ServiceInfo, settings: &LoggingSettings) -> Bo
     // NOTE: OXY-178, default is 128 (https://docs.rs/slog-async/2.7.0/src/slog_async/lib.rs.html#251)
     const CHANNEL_SIZE: usize = 1024;
 
+    // buffer json log lines up to 100 characters
+    const BUF_SIZE: usize = 100;
+
     let base_drain = match (&settings.output, &settings.format) {
         (LogOutput::Terminal, LogFormat::Text) => {
             let drain = TextDrain::new(TermDecorator::new().stdout().build())
@@ -78,7 +82,8 @@ pub(crate) fn init(service_info: &ServiceInfo, settings: &LoggingSettings) -> Bo
             AsyncDrain::new(drain).chan_size(CHANNEL_SIZE).build()
         }
         (LogOutput::Terminal, LogFormat::Json) => {
-            let drain = build_json_log_drain(io::stdout());
+            let buf = BufWriter::with_capacity(BUF_SIZE, io::stdout());
+            let drain = build_json_log_drain(buf);
             AsyncDrain::new(drain).chan_size(CHANNEL_SIZE).build()
         }
         (LogOutput::File(file), LogFormat::Text) => {
@@ -88,7 +93,8 @@ pub(crate) fn init(service_info: &ServiceInfo, settings: &LoggingSettings) -> Bo
             AsyncDrain::new(drain).chan_size(CHANNEL_SIZE).build()
         }
         (LogOutput::File(file), LogFormat::Json) => {
-            let drain = build_json_log_drain(File::create(file)?);
+            let buf = BufWriter::with_capacity(BUF_SIZE, File::create(file)?);
+            let drain = build_json_log_drain(buf);
             AsyncDrain::new(drain).chan_size(CHANNEL_SIZE).build()
         }
     };