Let's check /opt/splunk/var/log/splunk/splunkd.log for any indexing error like " metric event not indexed".
Also, can you try this curl from Linux host as well mentioned in doc: (https://docs.splunk.com/Documentation/Splunk/7.2.3/Data/Sendmetricstoametricsindex)
curl -k https://localhost:8088/services/collector \
-H "Authorization: Splunk b0221cd8-c4b4-465a-9a3c-273e3a75aa29" \
-d '{"time": 1486683865.000,"source":"disk","host":"host_99","fields":{"region":"us-west-1","datacenter":"us-west-1a","rack":"63","os":"Ubuntu16.10","arch":"x64","team":"LON","service":"6","service_version":"0","service_environment":"test","path":"/dev/sda1","fstype":"ext3","_value":1099511627776,"metric_name":"total"}}'
Replace token with yours and localhost with your SAI instance. See, if you get success.
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