Fix missing braces around initializer for std array

[SergeyRyabinin]

Issue #, if available:

HttpClientMetrics.cpp:44:9: error: missing braces around initializer for 
'std::__array_traits<std::pair<Aws::Monitoring::HttpClientMetricsType, const char*>, 14>::_Type 
{aka std::pair<Aws::Monitoring::HttpClientMetricsType, const char*> [14]}'
[-Werror=missing-braces]}; 

Description of changes:
Add more braces.
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