Add goimports and golangci-lint (open-telemetry#17)

goimports for import rewritting
golangci-lint as the configurable linting swiss army knife.

These tools are recorded in [tools.go](https://github.com/golang/go/wiki/Modules#how-can-i-track-tool-dependencies-for-a-module). This records
them as a dependency to make sure we're all using the same tool versions.

To make sure this project's tool's versions don't stomp over versions
from other projects, they are installed in ./.tools, which is
.gitignored.

goimports was run and fixed up a single file:
plugin/httptrace/httptrace.go

I prefer to group local packages below external packages, hence the use
of goimports -local option.

.golangci.yml contains nothing but an incomplete set of defaults ATM.
I expect those to change over time though. ;-)

To use, run:

$ make precommit

Fixes open-telemetry#15

.gitignore

@@ -0,0 +1 @@
+.tools/

.golangci.yml

@@ -0,0 +1,4 @@
+# See https://github.com/golangci/golangci-lint#config-file
+run:
+  issues-exit-code: 1 #Default
+  tests: true #Default

Makefile

@@ -0,0 +1,13 @@
+.PHONY: precommit
+
+TOOLS_DIR := ./.tools
+
+$(TOOLS_DIR)/golangci-lint: go.mod go.sum tools.go
+	go build -o $(TOOLS_DIR)/golangci-lint github.com/golangci/golangci-lint/cmd/golangci-lint
+
+$(TOOLS_DIR)/goimports: go.mod go.sum tools.go
+	go build -o $(TOOLS_DIR)/goimports golang.org/x/tools/cmd/goimports
+
+precommit: $(TOOLS_DIR)/goimports $(TOOLS_DIR)/golangci-lint 
+	$(TOOLS_DIR)/goimports -d -local github.com/open-telemetry/opentelemetry-go -w .
+	$(TOOLS_DIR)/golangci-lint run

go.mod

@@ -3,7 +3,9 @@ module github.com/open-telemetry/opentelemetry-go
 go 1.12
 
 require (
+	github.com/golangci/golangci-lint v1.17.1
 	github.com/lightstep/tracecontext.go v0.0.0-20181129014701-1757c391b1ac
 	github.com/onsi/ginkgo v1.8.0 // indirect
 	github.com/onsi/gomega v1.5.0 // indirect
+	golang.org/x/tools v0.0.0-20190521203540-521d6ed310dd
 )

plugin/httptrace/httptrace.go

@@ -18,10 +18,11 @@ import (
 	"encoding/binary"
 	"net/http"
 
-	"github.com/open-telemetry/opentelemetry-go/api/core"
-	"github.com/open-telemetry/opentelemetry-go/api/tag"
 	"github.com/lightstep/tracecontext.go"
 	"github.com/lightstep/tracecontext.go/tracestate"
+
+	"github.com/open-telemetry/opentelemetry-go/api/core"
+	"github.com/open-telemetry/opentelemetry-go/api/tag"
 )
 
 const (

tools.go

@@ -0,0 +1,8 @@
+// +build tools
+
+package tools
+
+import (
+	_ "github.com/golangci/golangci-lint/cmd/golangci-lint"
+	_ "golang.org/x/tools/cmd/goimports"
+)