Fix doc build issues happening in linux/memory (#27381)

* fix doc build issues

* add changelog

* move tests to linux-only

CHANGELOG.next.asciidoc

@@ -197,6 +197,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Improve `perfmon` metricset performance. {pull}26886[26886]
 - Preserve annotations in a kubernetes namespace metadata {pull}27045[27045]
 - Allow conditional processing in `decode_xml` and `decode_xml_wineventlog`. {pull}27159[27159]
+- Fix build constraint that caused issues with doc builds. {pull}27381[27381]
 
 *Auditbeat*
 

metricbeat/internal/metrics/memory/memory_helpers.go

@@ -0,0 +1,79 @@
+// Licensed to Elasticsearch B.V. under one or more contributor
+// license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright
+// ownership. Elasticsearch B.V. licenses this file to you under
+// the Apache License, Version 2.0 (the "License"); you may
+// not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+package memory
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"io/ioutil"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+// ParseMeminfo parses the contents of /proc/meminfo into a hashmap
+func ParseMeminfo(rootfs string) (map[string]uint64, error) {
+	table := map[string]uint64{}
+
+	meminfoPath := filepath.Join(rootfs, "/proc/meminfo")
+	err := readFile(meminfoPath, func(line string) bool {
+		fields := strings.Split(line, ":")
+
+		if len(fields) != 2 {
+			return true // skip on errors
+		}
+
+		valueUnit := strings.Fields(fields[1])
+		value, err := strconv.ParseUint(valueUnit[0], 10, 64)
+		if err != nil {
+			return true // skip on errors
+		}
+
+		if len(valueUnit) > 1 && valueUnit[1] == "kB" {
+			value *= 1024
+		}
+		table[fields[0]] = value
+
+		return true
+	})
+	return table, err
+}
+
+func readFile(file string, handler func(string) bool) error {
+	contents, err := ioutil.ReadFile(file)
+	if err != nil {
+		return errors.Wrapf(err, "error reading file %s", file)
+	}
+
+	reader := bufio.NewReader(bytes.NewBuffer(contents))
+
+	for {
+		line, _, err := reader.ReadLine()
+		if err == io.EOF {
+			break
+		}
+		if !handler(string(line)) {
+			break
+		}
+	}
+
+	return nil
+}

metricbeat/internal/metrics/memory/memory_linux.go

@@ -18,14 +18,6 @@
 package memory
 
 import (
-	"bufio"
-	"bytes"
-	"io"
-	"io/ioutil"
-	"path/filepath"
-	"strconv"
-	"strings"
-
 	"github.com/pkg/errors"
 
 	"github.com/elastic/beats/v7/libbeat/opt"
@@ -92,52 +84,3 @@ func get(rootfs string) (Memory, error) {
 	return memData, nil
 
 }
-
-// ParseMeminfo parses the contents of /proc/meminfo into a hashmap
-func ParseMeminfo(rootfs string) (map[string]uint64, error) {
-	table := map[string]uint64{}
-
-	meminfoPath := filepath.Join(rootfs, "/proc/meminfo")
-	err := readFile(meminfoPath, func(line string) bool {
-		fields := strings.Split(line, ":")
-
-		if len(fields) != 2 {
-			return true // skip on errors
-		}
-
-		valueUnit := strings.Fields(fields[1])
-		value, err := strconv.ParseUint(valueUnit[0], 10, 64)
-		if err != nil {
-			return true // skip on errors
-		}
-
-		if len(valueUnit) > 1 && valueUnit[1] == "kB" {
-			value *= 1024
-		}
-		table[fields[0]] = value
-
-		return true
-	})
-	return table, err
-}
-
-func readFile(file string, handler func(string) bool) error {
-	contents, err := ioutil.ReadFile(file)
-	if err != nil {
-		return errors.Wrapf(err, "error reading file %s", file)
-	}
-
-	reader := bufio.NewReader(bytes.NewBuffer(contents))
-
-	for {
-		line, _, err := reader.ReadLine()
-		if err == io.EOF {
-			break
-		}
-		if !handler(string(line)) {
-			break
-		}
-	}
-
-	return nil
-}

metricbeat/module/linux/memory/data.go

@@ -15,8 +15,6 @@
 // specific language governing permissions and limitations
 // under the License.
 
-// +build linux
-
 package memory
 
 import (
@@ -139,7 +137,7 @@ func GetVMStat() (*sysinfotypes.VMStatInfo, error) {
 	}
 	vmstatHandle, ok := h.(sysinfotypes.VMStat)
 	if !ok {
-		return nil, errors.Wrap(err, "VMStat not available for platform")
+		return nil, errors.New("VMStat not available for platform")
 	}
 	info, err := vmstatHandle.VMStat()
 	if err != nil {

metricbeat/module/linux/memory/memory.go

@@ -15,8 +15,6 @@
 // specific language governing permissions and limitations
 // under the License.
 
-// +build linux
-
 package memory
 
 import (

metricbeat/module/linux/memory/memory_test.go → metricbeat/module/linux/memory/memory_linux_test.go

@@ -15,8 +15,6 @@
 // specific language governing permissions and limitations
 // under the License.
 
-// +build linux
-
 package memory
 
 import (