Add suffixes to diagnostics and other cleanup (bevyengine#1505)

Also a few related clean ups to diagnostics
Old look (from the log_diagnostics_example):
<details>
<summary>Old look hidden to avoid clutter </summary>

![image](https://user-images.githubusercontent.com/36049421/108776774-bc349080-755a-11eb-8569-d4832abf6bc3.png)

</details>
New look: 

![image](https://user-images.githubusercontent.com/36049421/108776587-82638a00-755a-11eb-96eb-539026d59bcb.png)

In particular, notice that the width of the diagnostics has been significantly reduced - within vscode the value no longer wraps on my 1920 width monitor. The value is still 105 columns wide, so there is room for improvement however.

crates/bevy_asset/src/diagnostic/asset_count_diagnostics_plugin.rs

@@ -24,7 +24,7 @@ impl<T: Asset> AssetCountDiagnosticsPlugin<T> {
     pub fn setup_system(mut diagnostics: ResMut<Diagnostics>) {
         diagnostics.add(Diagnostic::new(
             Self::diagnostic_id(),
-            &format!("asset_count {}", std::any::type_name::<T>()),
+            format!("asset_count {}", std::any::type_name::<T>()),
             20,
         ));
     }

crates/bevy_diagnostic/src/diagnostic.rs

@@ -1,5 +1,8 @@
+use bevy_log::warn;
 use bevy_utils::{Duration, Instant, StableHashMap, Uuid};
-use std::collections::VecDeque;
+use std::{borrow::Cow, collections::VecDeque};
+
+use crate::log_diagnostics_plugin::MAX_LOG_NAME_WIDTH;
 
 /// Unique identifier for a [Diagnostic]
 #[derive(Debug, Copy, Clone, Hash, Eq, PartialEq, PartialOrd, Ord)]
@@ -29,7 +32,8 @@ pub struct DiagnosticMeasurement {
 #[derive(Debug)]
 pub struct Diagnostic {
     pub id: DiagnosticId,
-    pub name: String,
+    pub name: Cow<'static, str>,
+    pub suffix: Cow<'static, str>,
     history: VecDeque<DiagnosticMeasurement>,
     sum: f64,
     max_history_length: usize,
@@ -49,16 +53,35 @@ impl Diagnostic {
             .push_front(DiagnosticMeasurement { time, value });
     }
 
-    pub fn new(id: DiagnosticId, name: &str, max_history_length: usize) -> Diagnostic {
+    pub fn new(
+        id: DiagnosticId,
+        name: impl Into<Cow<'static, str>>,
+        max_history_length: usize,
+    ) -> Diagnostic {
+        let name = name.into();
+        if name.chars().count() > MAX_LOG_NAME_WIDTH {
+            // This could be a false positive due to a unicode width being shorter
+            warn!(
+                "Diagnostic {:?} has name longer than {} characters, and so might overflow in the LogDiagnosticsPlugin\
+                Consider using a shorter name.",
+                name, MAX_LOG_NAME_WIDTH
+            )
+        }
         Diagnostic {
             id,
-            name: name.to_string(),
+            name,
+            suffix: Cow::Borrowed(""),
             history: VecDeque::with_capacity(max_history_length),
             max_history_length,
             sum: 0.0,
         }
     }
 
+    pub fn with_suffix(mut self, suffix: impl Into<Cow<'static, str>>) -> Self {
+        self.suffix = suffix.into();
+        self
+    }
+
     pub fn value(&self) -> Option<f64> {
         self.history.back().map(|measurement| measurement.value)
     }

crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -27,7 +27,7 @@ impl FrameTimeDiagnosticsPlugin {
         DiagnosticId::from_u128(73441630925388532774622109383099159699);
 
     pub fn setup_system(mut diagnostics: ResMut<Diagnostics>) {
-        diagnostics.add(Diagnostic::new(Self::FRAME_TIME, "frame_time", 20));
+        diagnostics.add(Diagnostic::new(Self::FRAME_TIME, "frame_time", 20).with_suffix("s"));
         diagnostics.add(Diagnostic::new(Self::FPS, "fps", 20));
         diagnostics.add(Diagnostic::new(Self::FRAME_COUNT, "frame_count", 1));
     }

crates/bevy_diagnostic/src/log_diagnostics_plugin.rs

@@ -28,6 +28,10 @@ impl Default for LogDiagnosticsPlugin {
     }
 }
 
+/// The width which diagnostic names will be printed as
+/// Plugin names should not be longer than this value
+pub(crate) const MAX_LOG_NAME_WIDTH: usize = 32;
+
 impl Plugin for LogDiagnosticsPlugin {
     fn build(&self, app: &mut bevy_app::AppBuilder) {
         app.insert_resource(LogDiagnosticsState {
@@ -58,11 +62,25 @@ impl LogDiagnosticsPlugin {
         if let Some(value) = diagnostic.value() {
             if let Some(average) = diagnostic.average() {
                 info!(
-                    "{:<65}: {:<10.6}  (avg {:.6})",
-                    diagnostic.name, value, average
+                    target: "bevy diagnostic",
+                    "{:<name_width$}: {:>12} (avg {:>})",
+                    diagnostic.name,
+                    // Suffix is only used for 's' as in seconds currently,
+                    // so we reserve one column for it
+                    format!("{:.6}{:1}", value, diagnostic.suffix),
+                    // Do not reserve one column for the suffix in the average
+                    // The ) hugging the value is more aesthetically pleasing
+                    format!("{:.6}{:}", average, diagnostic.suffix),
+                    name_width = MAX_LOG_NAME_WIDTH,
                 );
             } else {
-                info!("{:<65}: {:<10.6}", diagnostic.name, value);
+                info!(
+                    target: "bevy diagnostic",
+                    "{:<name_width$}: {:>}",
+                    diagnostic.name,
+                    format!("{:.6}{:}", value, diagnostic.suffix),
+                    name_width = MAX_LOG_NAME_WIDTH,
+                );
             }
         }
     }