Protobuf Collector fixes:
1. Elemental fields at top level in a protobuf message need
   to be treated as tags. Add remote UDP endpoint IP as Source tag.
2. Add static schema for PFEHeapInfo in viz.sandesh so that
   WebUI can display graphs of statistics sent from MX.
3. Modify tests as per 1 above.

Change-Id: I2ae6a31b885523a6875e961e8bc9176937612883
Merge "Protobuf Collector fixes: 1. Elemental fields at top level in a protobuf message need    to be treated as tags. Add remote UDP endpoint IP as Source tag. 2. Add static schema for PFEHeapInfo in viz.sandesh so that    WebUI can display graphs of statistics sent from MX. 3. Modify tests as per 1 above."