FIX: Enrich context before collecting properties

LogCompletion event was not including properties added via
`EnrichDiagnosticContext` delegate on options because
the properties had been collected before invoking the enricher.

Swapping the order of the statements fixes this.

Author: alastair-gatpark

src/Serilog.AspNetCore/AspNetCore/RequestLoggingMiddleware.cs

@@ -78,12 +78,12 @@ bool LogCompletion(HttpContext httpContext, DiagnosticContextCollector collector
 
             if (!logger.IsEnabled(level)) return false;
 
-            if (!collector.TryComplete(out var collectedProperties))
-                collectedProperties = NoProperties;
-
             // Enrich diagnostic context
             _enrichDiagnosticContext?.Invoke(_diagnosticContext, httpContext);
 
+            if (!collector.TryComplete(out var collectedProperties))
+                collectedProperties = NoProperties;
+
             // Last-in (correctly) wins...
             var properties = collectedProperties.Concat(new[]
             {