bugfix: add missing LICENSE in vendor

[fuweid]

Signed-off-by: Wei Fu [email protected]

Ⅰ. Describe what this PR did

github.com/prometheus/client_golang vendor package miss the LICENSE. We
should add it back.

Ⅱ. Does this pull request fix one issue?

NONE

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

NONE

Ⅳ. Describe how to verify it

# -> before 
# vagrant @ ubuntu-xenial in ~/go/src/github.com/alibaba/pouch on git:master o [13:19:02]
$ govendor status
The following packages are missing or modified locally:
        github.com/prometheus/client_golang
Error: status failed for 1 package(s)

# <- after
# vagrant @ ubuntu-xenial in ~/go/src/github.com/alibaba/pouch on git:bugfix_make_govendor_status_correct o [13:25:55]
$ govendor status

# vagrant @ ubuntu-xenial in ~/go/src/github.com/alibaba/pouch on git:bugfix_make_govendor_status_correct o [13:33:09]
$ echo $?
0

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Merging #2418 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2418      +/-   ##
==========================================
- Coverage   68.45%   68.38%   -0.08%     
==========================================
  Files         275      275              
  Lines       18291    18291              
==========================================
- Hits        12522    12509      -13     
- Misses       4340     4354      +14     
+ Partials     1429     1428       -1

Flag	Coverage Δ
#criv1alpha1test	31.82% <ø> (+0.01%)	⬆️
#criv1alpha2test	35.69% <ø> (-0.14%)	⬇️
#integrationtest	39.75% <ø> (-0.02%)	⬇️
#nodee2etest	33.2% <ø> (-0.09%)	⬇️
#unittest	25.52% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
cri/ocicni/cni_manager.go	59.18% <0%> (-12.25%)	⬇️
daemon/mgr/system.go	67.93% <0%> (-5.35%)	⬇️
ctrd/image.go	76.75% <0%> (-2.2%)	⬇️
daemon/containerio/container_io.go	74.86% <0%> (-1.1%)	⬇️
cri/v1alpha2/cri.go	67.69% <0%> (-0.6%)	⬇️
cri/v1alpha2/cri_utils.go	90.51% <0%> (-0.29%)	⬇️
cri/v1alpha2/cri_wrapper.go	61.2% <0%> (ø)	⬆️
ctrd/container.go	60.23% <0%> (+0.47%)	⬆️
daemon/mgr/container_utils.go	86.07% <0%> (+0.63%)	⬆️
daemon/mgr/container.go	60.23% <0%> (+0.65%)	⬆️
... and 2 more

[allencloud]

LGTM