add MeterDataCollection (#29)

diff --git a/language-agent/Meter.proto b/language-agent/Meter.proto
index 82c2c44..b27b902 100644
--- a/language-agent/Meter.proto
+++ b/language-agent/Meter.proto
@@ -79,3 +79,7 @@
     // Meter data report time, be set value in the first element in the stream-call.
     int64 timestamp = 5;
 }
+
+message MeterDataCollection {
+    repeated MeterData meterData = 1;
+}
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