Fix Auto metrics relabeled errors

[weenxin]

fix #592

[codecov]

Codecov Report

Merging #593 into master will decrease coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #593      +/-   ##
==========================================
- Coverage   68.02%   67.89%   -0.13%     
==========================================
  Files         154      154              
  Lines       21225    21228       +3     
==========================================
- Hits        14438    14413      -25     
- Misses       5624     5656      +32     
+ Partials     1163     1159       -4     

Impacted Files	Coverage Δ
lib/promscrape/scrapework.go	72.84% <100.00%> (+0.55%)	⬆️
lib/mergeset/block_stream_reader.go	50.96% <0.00%> (-11.62%)	⬇️
lib/storage/merge.go	76.85% <0.00%> (-3.31%)	⬇️
lib/mergeset/table.go	66.75% <0.00%> (-1.54%)	⬇️
lib/storage/index_db.go	76.03% <0.00%> (+0.16%)	⬆️
app/vmselect/promql/aggr_incremental.go	91.40% <0.00%> (+0.68%)	⬆️

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[valyala]

LGTM

[valyala]

@weenxin , thanks for the contribution!