[splunkhecexporter]Add metric_type as key which maps to the type of the metric

[Saber-W]

Description:
Currently, some metadata isn't stored into Splunk HEC when converting. This PR adds metric_type to the fields of Splunk event to transfer this info. The type of metric could be one of: gauge, sum, histogram or summary.

Link to tracking Issue:
#3289

Testing:
Update related unit testing Test_metricDataToSplunk.

[Saber-W]

cc @bogdandrutu @tigrannajaryan @Aneurysm9 @anuraaga @djaglowski @dmitryax @owais

[tigrannajaryan]

@Saber-W lint job failed. Please make sure the build passes.

make[1]: Leaving directory '/home/circleci/project/exporter/sentryexporter'
make -C ./exporter/splunkhecexporter lint
make[1]: Entering directory '/home/circleci/project/exporter/splunkhecexporter'
golangci-lint run --allow-parallel-runners
metricdata_to_splunk_test.go:510: File is not `gofmt`-ed with `-s` (gofmt)
						"metric_type": "IntSum",
make[1]: *** [../../Makefile.Common:93: lint] Error 1
make[1]: Leaving directory '/home/circleci/project/exporter/splunkhecexporter'
make: *** [Makefile:149: for-all-target-./exporter/splunkhecexporter] Error 2
make: *** Waiting for unfinished jobs....
make[1]: Leaving directory '/home/circleci/project/exporter/signalfxexporter'

Exited with code exit status 2
CircleCI received exit code 2

[Saber-W]

Hi @tigrannajaryan, sure. It's the gofmt issue, which should be fixed now.

[atoulme]

LGTM thanks for the contrib!

[tigrannajaryan]

Thank you @Saber-W

[Saber-W]

Thanks!