Add estimator interface and global metric estimator implementation

docs/src/lib/internals/estimator.md

@@ -6,6 +6,16 @@ The interface that has to be implemented for an estimator.
 
 ```@autodocs
 Modules = [MetagraphOptimization]
-Pages = ["models/estimator.jl"]
+Pages = ["estimator/interafce.jl"]
 Order = [:type, :constant, :function]
 ```
+
+## Global Metric Estimator
+
+Implementation of a global metric estimator. It uses the graph properties compute effort, data transfer, and compute intensity.
+
+```@autodocs
+Modules = [MetagraphOptimization]
+Pages = ["estimator/global_metric.jl"]
+Order = [:type, :function]
+```