Reuse StringBuilders in EventArgsFormatting

While I was editing in this area I noticed that the calling pattern
around StringBuilders in FormatEventMessage looked allocatey.

Instead of creating two throwaway StringBuilders to format a single
message,

1. Grab a ReusableStringBuilder
2. Reuse the builder between "construct format string" and "get final
   message".

We chose ReusableStringBuilder over StringBuilderCache because logging
sometimes creates strings that are _much_ larger than the 512 character
limit of SBC. That also reduces the need to prereserve a size: the
process-wide pool's elements should be pretty big already.

See dotnet#2697 (comment)
for stats on string length.

src/Shared/EventArgsFormatting.cs

@@ -219,7 +219,9 @@ internal static string FormatEventMessage
             string logOutputProperties
         )
         {
-            StringBuilder format = new StringBuilder();
+            // capacity is the longest possible path through the below
+            // to avoid reallocating while constructing the string
+            using ReuseableStringBuilder format = new(51);
 
             // Uncomment these lines to show show the processor, if present.
             /*
@@ -328,9 +330,11 @@ string logOutputProperties
 
             string finalFormat = format.ToString();
 
+            // Reuse the string builder to create the final message
+            ReuseableStringBuilder formattedMessage = format.Clear();
+
             // If there are multiple lines, show each line as a separate message.
             string[] lines = SplitStringOnNewLines(message);
-            StringBuilder formattedMessage = new StringBuilder();
 
             for (int i = 0; i < lines.Length; i++)
             {