Fix reading old samples missing slabinfo

Summary:
Slabinfo was added as non-optional. When it's missing (e.g. from past
samples) below will fail reading the samples.

Instead if it's missing make it deserialize to default empty map.

Reviewed By: lnyng

Differential Revision: D53205190

fbshipit-source-id: 5d55c9faf5b90e7834c13f60f9b5b758fef7883e

below/model/src/sample.rs

@@ -62,6 +62,7 @@ pub struct SystemSample {
     pub stat: procfs::Stat,
     pub meminfo: procfs::MemInfo,
     pub vmstat: procfs::VmStat,
+    #[serde(default)]
     pub slabinfo: procfs::SlabInfoMap,
     pub hostname: String,
     pub disks: procfs::DiskMap,