add sqlserver input plugin

plugins/inputs/sqlserver/README.md

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+# SQL Server plugin
+
+This sqlserver plugin provides metrics for your SQL Server instance. 
+It currently works with SQL Server versions 2008+. 
+Recorded metrics are lightweight and use Dynamic Management Views supplied by SQL Server:
+```
+Performance counters  : 1000+ metrics from sys.dm_os_performance_counters
+Performance metrics   : some special performance metrics
+Wait stats 			  : list of wait tasks categorized from sys.dm_os_wait_stats
+Memory clerk		  : memory breakdown from sys.dm_os_memory_clerks
+Database size         : database size trend, data and log file from sys.dm_io_virtual_file_stats
+Database IO			  : database I/O from sys.dm_io_virtual_file_stats
+Database latency	  : database reads and writes latency from sys.dm_io_virtual_file_stats
+CPU				      : cpu usage from sys.dm_os_ring_buffers
+```
+
+You must create a login on every instance you want to monitor, with following script:
+```SQL 
+USE master; 
+GO
+CREATE LOGIN [telegraf] WITH PASSWORD = N'mystrongpassword';
+GO
+GRANT VIEW SERVER STATE TO [telegraf]; 
+GO
+GRANT VIEW ANY DEFINITION TO [telegraf]; 
+GO
+```

plugins/inputs/sqlserver/sqlserver_test.go

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+package sqlserver
+
+import (
+	"testing"
+
+	"github.com/influxdb/telegraf/testutil"
+	"github.com/stretchr/testify/require"
+)
+
+func TestSqlServerGeneratesMetrics(t *testing.T) {
+	if testing.Short() {
+		t.Skip("Skipping integration test in short mode")
+	}
+
+	s := &SqlServer{}
+	s.Servers = append(s.Servers, &Server{ConnectionString: "Server=192.168.1.30;User Id=linuxuser;Password=linuxuser;app name=telegraf;log=1;"})
+	
+	var acc testutil.Accumulator
+
+	err := s.Gather(&acc)
+	require.NoError(t, err)
+}