SQL Server 2017

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create table #consecutivedates (sgid nvarchar(max), metric nvarchar(max), targetvalue nvarchar(max), startdate datetime, enddate datetime)
insert into #consecutivedates values ('2177', '515818', '18', '2019-09-01',  '2019-09-30'),
('2177', '515818', '125', '2019-08-01',  '2019-08-31'),
('2177', '515818', '15', '2019-07-01',  '2019-07-31')
3 rows affected

select * from  #consecutivedates


sgid
metric
targetvalue
startdate
enddate


2177
515818
18
2019-09-01 00:00:00.000
2019-09-30 00:00:00.000

2177
515818
125
2019-08-01 00:00:00.000
2019-08-31 00:00:00.000

2177
515818
15
2019-07-01 00:00:00.000
2019-07-31 00:00:00.000


	select sgid, metric, min(startdate) startdate, max(enddate) enddate
	from (
		select
			t.*,
			sum(case when startdate = dateadd(day, 1, lag_enddate) then 0 else 1 end)
				over(partition by sgid, metric order by startdate) grp
		from (
			select 
				t.*, 
				lag(enddate) over(partition by sgid, metric order by startdate) lag_enddate
			from  #consecutivedates t
		) t
	) t
	group by sgid, metric, grp


sgid
metric
startdate
enddate


2177
515818
2019-07-01 00:00:00.000
2019-09-30 00:00:00.000

sgid	metric	targetvalue	startdate	enddate
2177	515818	18	2019-09-01 00:00:00.000	2019-09-30 00:00:00.000
2177	515818	125	2019-08-01 00:00:00.000	2019-08-31 00:00:00.000
2177	515818	15	2019-07-01 00:00:00.000	2019-07-31 00:00:00.000