counter: add dummy api for go1.18 or older

internal/counter and internal/mmap use features available
only after go1.19/go1.20. Gopls, the first application to
use this package, still supports builds with go1.18.
Add build constraints and a dummy file to avoid build breakage
when gopls starts to use this.

And delete interna/counter/bootstrap.go. This was intended
to support the go bootstrap build when the internal/counter
package was originally in the main Go project. The code is
now moved to x/telemetry, and the build constraint don't make
sense.

Change-Id: Ibd32c6edf9104cfab197272808a701ddd2ff3bb3
Reviewed-on: https://go-review.googlesource.com/c/telemetry/+/504955
Reviewed-by: Jamal Carvalho <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>

Author: hyangah

counter/counter.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.19
+
 package counter
 
 // The implementation of this package and tests are located in

internal/counter/bootstrap.go renamed to counter/counter_go118.go

@@ -2,10 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build compiler_bootstrap
+//go:build !go1.19
 
 package counter
 
+import "fmt"
+
 func Add(string, int64) {}
 func Inc(string)        {}
 func Open()             {}
@@ -23,3 +25,7 @@ type File struct {
 }
 
 func Parse(filename string, data []byte) (*File, error) { return nil, fmt.Errorf("unimplemented") }
+
+type StackCounter struct{ name string }
+
+func NewStack(name string, _ int) *StackCounter { return &StackCounter{name} }

go.mod

@@ -3,4 +3,5 @@ module golang.org/x/telemetry
 go 1.20
 
 require golang.org/x/mod v0.10.0
+
 require golang.org/x/sys v0.8.0