SF-3920 Fix event metrics not being recorded for BuildCompletedAsync

[pmachapman]

This PR fixes a bug I noticed on live where the following error was being logged in the journal:

Error logging event metric for BuildCompletedAsync
System.FormatException: Cannot deserialize a 'BsonDocument' from BsonType 'Int32'.
   at MongoDB.Bson.Serialization.Serializers.BsonValueSerializerBase`1.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   at MongoDB.Bson.BsonDocument.Parse(String json)
   at SIL.XForge.Services.EventMetricService.GetBsonValue(Object objectValue) in D:\Source\Repos\web-xforge\src\SIL.XForge\Services\EventMetricService.cs:line 175
   at SIL.XForge.Services.EventMetricService.SaveEventMetricAsync(String projectId, String userId, String eventType, EventScope scope, Dictionary`2 argumentsWithNames, Object result, Nullable`1 executionTime, Exception exception) in D:\Source\Repos\web-xforge\src\SIL.XForge\Services\EventMetricService.cs:line 108
   at SIL.XForge.EventMetrics.EventMetricLogger.<>c__DisplayClass7_0.<<Intercept>g__SaveEventMetricAsync|0>d.MoveNext() in D:\Source\Repos\web-xforge\src\SIL.XForge\EventMetrics\EventMetricLogger.cs:line 189

This error doesn't prevent builds from completing, but will prevent the recording of the BuildCompletedAsync event metric.

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.26%. Comparing base (b906996) to head (c130697).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4079   +/-   ##
=======================================
  Coverage   81.25%   81.26%           
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  Files         670      670           
  Lines       43467    43473    +6     
  Branches     7132     7126    -6     
=======================================
+ Hits        35321    35328    +7     
+ Misses       6976     6960   -16     
- Partials     1170     1185   +15     

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