update protobuf to version 3

backend/storage/storage.proto

+syntax = "proto3";
+package storage;
+
+enum DataType {
+    // single entry
+    kInt32 = 0;
+    kInt64 = 1;
+    kFloat = 2;
+    kDouble = 3;
+    kString = 4;
+    kBool = 5;
+    // entrys
+    kInt64s = 6;
+    kFloats = 7;
+    kDoubles = 8;
+    kStrings = 9;
+    kInt32s = 10;
+    kBools = 11;
+}
+
+// A data array, which type is `type`.
+message Entry {
+    // if all the entries in a record share the same data type, ignore this field
+    // and store type to `dtype` in `Record`.
+    DataType dtype = 1;
+    // single element
+    int32 i32 = 2;
+    int64 i64 = 3;
+    string s = 4;
+    float f = 5;
+    double d = 6;
+    bool b = 7;
+    // array
+    repeated int64 i64s = 8;
+    repeated float fs = 9;
+    repeated double ds = 10;
+    repeated int32 i32s = 11;
+    repeated bool bs = 12;
+}
+
+message Record {
+    Entry data = 1;
+    int64 timestamp = 2;
+    // store the count of writing operations to the tablet.
+    int64 id = 3;
+    DataType dtype = 4;
+    // shape or some other meta infomation for this record, if all the records
+    // share the same meta, just store one copy of meta in `Storage`, or create
+    // a unique copy for each `Record`.
+    Entry meta = 5;
+}
+
+/*
+A Tablet stores the records of a component which type is `component` and indidates as `tag`.
+The records will be saved in a file which name contains `tag`. During the running period,
+`num_records` will be accumulated, and `num_samples` indicates the size of sample set the
+reservoir sampling algorithm will collect.
+*/
+message Tablet {
+    // the kinds of the components that supported
+    enum Type {
+        kScalar = 0;
+        kHistogram = 1;
+        kGraph = 2;
+    }
+    // type of the component, different component should have different storage format.
+    Type component = 1;
+    // records the total count of records, each Write operation should increate this value.
+    int64 num_records = 2;
+    // indicate the number of instances to sample, this should be a constant value.
+    int32 num_samples = 3;
+    repeated Record records = 4;
+    // store a meta infomation if all the records share.
+    Entry meta = 5;
+    // the unique identification for this `Tablet`.
+    string tag = 6;
+}
+
+/*
+The Storage stores all the records.
+*/
+message Storage {
+    // tags to Tablet, should be thread safe if fix the keys after initialization.
+    map<string, Tablet> tablets = 1;
+    string dir = 2;
+    int64 timestamp = 3;
+}
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