[Internal] Performance: Fixes exception serialization when tracing is not enabled

[ealsur]

When an Exception travels through the ResourceThrottleRetryPolicy.ShouldRetry, and the error is not one the current policy handles, we trace it like so:

DefaultTrace.TraceError(
        "Operation will NOT be retried. Current attempt {0}, Exception: {1} ",
        this.currentAttemptCount,
        this.GetExceptionMessage(exception));

DefaultTrace.TraceError has this signature:

TraceError(string format, params object[] args)

So the object[] ToString invocation on each of the arguments is only called if tracing is enabled.

But our current code on this.GetExceptionMessage calls exception.ToString() so it is serializing the exception even if tracing is not enabled (because it needs to construct the object array).

This PR defers the ToString call to only be invoked if there is tracing enabled.

Normal 429s (Throttles) are exceptionless, so this issue surfaces only in cases where there is another exception happening on the transport path.

[ghost]

Closing due to in-activity, pease feel free to re-open.