[chore] Fix make generate on Windows (open-telemetry#43509)

Fix the `generate` target so developers using Windows can do `make
generate` for their components on Windows. Main things in this change:

1. Use of path that can be pre-pended to `PATH`, the one being used
contains `:` and even escaping it is not enough to get it to correctly
work when it is being included in `PATH`
2. Ensure that on Windows all tools used by `go generate` have the `exe`
extension. This is needed because the `cmd` package used by `go
generate` checks for the known executable extensions on Windows. Notice
that `make` and Git Bash don't require that and same extension is not
needed for the other tools.
3. The `generate` target now triggers building the tools it needs as
various other targets so it can avoid the full `install-tools`.

Future work:
- `go.*` targets are really slow on Windows since they trigger the
re-evaluation of all shell commands that are really expensive on
Windows. Basically any recursive make call from `for-all` is currently
too expensive on Windows.
- Move the definition of components custom tools to the respective
components, possibly using the new go.mod `tool` directive. For now I
opted to keep the tool used by `githubreceiver` as it is.

Author: pjanotti

Makefile

@@ -222,7 +222,7 @@ goporto: $(PORTO)
 
 .PHONY: for-all
 for-all:
-	@set -e; for dir in $(ALL_MODS); do \
+	@set -e; for dir in $${ALL_MODS}; do \
 	  (cd "$${dir}" && \
 	  	echo "running $${CMD} in $${dir}" && \
 	 	$${CMD} ); \

Makefile.Common

@@ -90,6 +90,11 @@ GOTESTSUM           := $(TOOLS_BIN_DIR)/gotestsum
 CHECKAPI            := $(TOOLS_BIN_DIR)/checkapi
 MODERNIZE           := $(TOOLS_BIN_DIR)/modernize
 
+# Tools used by go generate commands
+MDATAGEN := $(TOOLS_BIN_DIR)/mdatagen
+# The githubreceiver uses a "go generate" tool that is not used by other components.
+GENQLIENT := $(TOOLS_BIN_DIR)/genqlient
+
 GOTESTSUM_OPT?= --rerun-fails=1
 
 # BUILD_TYPE should be one of (dev, release).
@@ -244,13 +249,31 @@ fmt: $(GOFUMPT) $(GOIMPORTS)
 	$(GOFUMPT) -l -w -extra .
 	$(GOIMPORTS) -w -local github.com/open-telemetry/opentelemetry-collector-contrib ./
 
+.PHONY: generate-tools
+generate-tools: $(MDATAGEN) $(GENQLIENT)
+ifeq ($(GOOS),windows)
+	@echo Adding "exe" extensions to generate tools
+	@for f in $^; do \
+		cp -u "$$f" "$$f.exe"; \
+	done
+endif
+
+# On Windows changing path with absolute path includes ':' even escaping it is not enough to get it to correctly
+# update PATH, so use a relative path when pre-pending to PATH on Windows. Unfortunately, realpath is not portable
+# between mac and GNU based bash so keeping the full path on non-Windows OSes.
+ifeq ($(GOOS),windows)
+    TOOLS_BIN_DIR_PORTABLE?=$(shell realpath --relative-to="$(CURDIR)" "$(TOOLS_BIN_DIR)")
+else
+    TOOLS_BIN_DIR_PORTABLE?=$(TOOLS_BIN_DIR)
+endif
+
 .PHONY: generate
-generate: install-tools
+generate: generate-tools
 ifeq ($(CURDIR),$(SRC_ROOT))
-	PATH="$(TOOLS_BIN_DIR):$$PATH" $(MAKE) for-all CMD="$(GOCMD) generate ./..."
+	PATH="$(TOOLS_BIN_DIR_PORTABLE):$$PATH" $(MAKE) for-all CMD="$(GOCMD) generate ./..."
 	$(MAKE) gofmt
 else
-	PATH="$(TOOLS_BIN_DIR):$$PATH" $(GOCMD) generate ./...
+	PATH="$(TOOLS_BIN_DIR_PORTABLE):$$PATH" $(GOCMD) generate ./...
 	$(MAKE) fmt
 endif