[metric-label-support]: Initial setup

[metric-label-support]: Initial setup

This is the initial commit on the proposal defined in the document
"Adding Labels to Collectd Metrics"
  - https://docs.google.com/document/d/173gGP3tUD3yfN2NNHxCv0BsKsacfDlSyoaQIn7MqLtQ/edit?usp=sharing
With the datastructures updated as per the document:
"Compare data structures for metric labels (collectd)"
  - https://docs.google.com/document/d/1RZ7YAO6lWcDDvevzxFGVDgv4UQpdgWoT7OCP9Vpb2CU/edit?usp=sharing

This commit adds the initial declaration and definition of the basic
data structure, and sets up the build system to add in unit testing
for the label store, the identity and metrics data streuctures.

Signed-off-by: Manoj Srivastava <srivasta@google.com>
Change-Id: I29673ab36f75db2ac487ee6a22907e3deba2f965