Proto: make badger/v2 compatible with v1 (#1293)

There were two instances of init-time work being incompatible with v1.
That is, if one imported both v1 and v2 as part of a Go build, the
resulting binary would end up panicking before main could run. The
examples below are done with the latest versions of v1 and v2, and a
main.go as follows:

	package main

	import (
		_ "github.com/dgraph-io/badger"
		_ "github.com/dgraph-io/badger/v2"
	)

	func main() {}

First, the protobuf package used "pb" as its proto package name. This is
a problem, because types are registered globally with their fully
qualified names, like "pb.Foo". Since both badger/pb and badger/v2/pb
tried to globally register types with the same qualified names, we'd get
a panic:

	$ go run .
	panic: proto: duplicate enum registered: pb.ManifestChange_Operation

	goroutine 1 [running]:
	github.com/golang/protobuf/proto.RegisterEnum(...)
		.../go/pkg/mod/github.com/golang/protobuf@v1.3.1/proto/properties.go:459
	github.com/dgraph-io/badger/v2/pb.init.0()
		.../badger/pb/pb.pb.go:638 +0x459

To fix this, make v2's proto package fully qualified. Since the
namespace is global, just "v2.pb" wouldn't suffice; it's not unique
enough. "dgraph.badger.v2.pb" seems good, since it follows the Go module
path pretty closely.

The second issue was with expvar, which too uses globally registered
names:

	$ go run .
	2020/04/08 22:59:20 Reuse of exported var name: badger_disk_reads_total
	panic: Reuse of exported var name: badger_disk_reads_total

	goroutine 1 [running]:
	log.Panicln(0xc00010de48, 0x2, 0x2)
		.../src/log/log.go:365 +0xac
	expvar.Publish(0x906fcc, 0x17, 0x9946a0, 0xc0000b0318)
		.../src/expvar/expvar.go:278 +0x267
	expvar.NewInt(...)
		.../src/expvar/expvar.go:298
	github.com/dgraph-io/badger/v2/y.init.1()
		.../badger/y/metrics.go:55 +0x65
	exit status 2

This time, replacing the "badger_" var prefix with "badger_v2_" seems
like it's simple enough as a fix.

Fixes #1208.

pb/pb.proto

@@ -17,7 +17,7 @@
 // Use protos/gen.sh to generate .pb.go files.
 syntax = "proto3";
 
-package pb;
+package dgraph.badger.v2.pb;
 
 option go_package = "github.com/dgraph-io/badger/v2/pb";
 

y/metrics.go

@@ -52,17 +52,17 @@ var (
 
 // These variables are global and have cumulative values for all kv stores.
 func init() {
-	NumReads = expvar.NewInt("badger_disk_reads_total")
-	NumWrites = expvar.NewInt("badger_disk_writes_total")
-	NumBytesRead = expvar.NewInt("badger_read_bytes")
-	NumBytesWritten = expvar.NewInt("badger_written_bytes")
-	NumLSMGets = expvar.NewMap("badger_lsm_level_gets_total")
-	NumLSMBloomHits = expvar.NewMap("badger_lsm_bloom_hits_total")
-	NumGets = expvar.NewInt("badger_gets_total")
-	NumPuts = expvar.NewInt("badger_puts_total")
-	NumBlockedPuts = expvar.NewInt("badger_blocked_puts_total")
-	NumMemtableGets = expvar.NewInt("badger_memtable_gets_total")
-	LSMSize = expvar.NewMap("badger_lsm_size_bytes")
-	VlogSize = expvar.NewMap("badger_vlog_size_bytes")
-	PendingWrites = expvar.NewMap("badger_pending_writes_total")
+	NumReads = expvar.NewInt("badger_v2_disk_reads_total")
+	NumWrites = expvar.NewInt("badger_v2_disk_writes_total")
+	NumBytesRead = expvar.NewInt("badger_v2_read_bytes")
+	NumBytesWritten = expvar.NewInt("badger_v2_written_bytes")
+	NumLSMGets = expvar.NewMap("badger_v2_lsm_level_gets_total")
+	NumLSMBloomHits = expvar.NewMap("badger_v2_lsm_bloom_hits_total")
+	NumGets = expvar.NewInt("badger_v2_gets_total")
+	NumPuts = expvar.NewInt("badger_v2_puts_total")
+	NumBlockedPuts = expvar.NewInt("badger_v2_blocked_puts_total")
+	NumMemtableGets = expvar.NewInt("badger_v2_memtable_gets_total")
+	LSMSize = expvar.NewMap("badger_v2_lsm_size_bytes")
+	VlogSize = expvar.NewMap("badger_v2_vlog_size_bytes")
+	PendingWrites = expvar.NewMap("badger_v2_pending_writes_total")
 }