Build examples last (open-telemetry#1370)

Examples are "leaf" packages, that pull some other packages. So build
the other packages first to get and fix all the errors there, before
building the examples.

Makefile

@@ -18,7 +18,7 @@ TOOLS_MOD_DIR := ./internal/tools
 # All source code and documents. Used in spell check.
 ALL_DOCS := $(shell find . -name '*.md' -type f | sort)
 # All directories with go.mod files related to opentelemetry library. Used for building, testing and linting.
-ALL_GO_MOD_DIRS := $(filter-out $(TOOLS_MOD_DIR), $(shell find . -type f -name 'go.mod' -exec dirname {} \; | sort))
+ALL_GO_MOD_DIRS := $(filter-out $(TOOLS_MOD_DIR), $(shell find . -type f -name 'go.mod' -exec dirname {} \; | egrep -v '^./example' | sort)) $(shell find ./example -type f -name 'go.mod' -exec dirname {} \; | sort)
 ALL_COVERAGE_MOD_DIRS := $(shell find . -type f -name 'go.mod' -exec dirname {} \; | egrep -v '^./example|^$(TOOLS_MOD_DIR)' | sort)
 
 # Mac OS Catalina 10.5.x doesn't support 386. Hence skip 386 test