[processor/transform] add Hour converter (open-telemetry#29743)

**Description:** Adds `Hour` converter

closes open-telemetry#29468 

**Testing:** adds unit test

**Documentation:** doc'd in readme for ottl converters

---------

Co-authored-by: Evan Bradley <11745660+evan-bradley@users.noreply.github.com>

.chloggen/hour-of-day.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: pkg/ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add Hour OTTL Converter
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [29468]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

pkg/ottl/ottlfuncs/README.md

@@ -305,6 +305,7 @@ Available Converters:
 - [ConvertCase](#convertcase)
 - [ExtractPatterns](#extractpatterns)
 - [FNV](#fnv)
+- [Hour](#hour)
 - [Hours](#hours)
 - [Double](#double)
 - [Duration](#duration)
@@ -454,6 +455,20 @@ Examples:
 
 - `FNV("name")`
 
+### Hour
+
+`Hour(value)`
+
+The `Hour` Converter returns the hour from the specified time.  The Converter [uses the `time.Hour` function](https://pkg.go.dev/time#Time.Hour).
+
+`value` is a `time.Time`. If `value` is another type an error is returned.
+
+The returned type is `int64`.
+
+Examples:
+
+- `Hour(Now())`
+
 ### Hours
 
 `Hours(value)`

pkg/ottl/ottlfuncs/func_hour.go

@@ -0,0 +1,38 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package ottlfuncs // import "github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl/ottlfuncs"
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+type HourArguments[K any] struct {
+	Time ottl.TimeGetter[K]
+}
+
+func NewHourFactory[K any]() ottl.Factory[K] {
+	return ottl.NewFactory("Hour", &HourArguments[K]{}, createHourFunction[K])
+}
+func createHourFunction[K any](_ ottl.FunctionContext, oArgs ottl.Arguments) (ottl.ExprFunc[K], error) {
+	args, ok := oArgs.(*HourArguments[K])
+
+	if !ok {
+		return nil, fmt.Errorf("HourFactory args must be of type *HourArguments[K]")
+	}
+
+	return Hour(args.Time)
+}
+
+func Hour[K any](t ottl.TimeGetter[K]) (ottl.ExprFunc[K], error) {
+	return func(ctx context.Context, tCtx K) (any, error) {
+		time, err := t.Get(ctx, tCtx)
+		if err != nil {
+			return nil, err
+		}
+		return int64(time.Hour()), nil
+	}, nil
+}

pkg/ottl/ottlfuncs/func_hour_test.go

@@ -0,0 +1,54 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package ottlfuncs
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/open-telemetry/opentelemetry-collector-contrib/pkg/ottl"
+)
+
+func Test_Hour(t *testing.T) {
+	tests := []struct {
+		name     string
+		time     ottl.TimeGetter[any]
+		expected int64
+	}{
+		{
+			name: "some time",
+			time: &ottl.StandardTimeGetter[any]{
+				Getter: func(ctx context.Context, tCtx any) (any, error) {
+					return time.Date(2006, time.January, 2, 15, 4, 5, 0, time.UTC), nil
+				},
+			},
+			expected: 15,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			exprFunc, err := Hour(tt.time)
+			assert.NoError(t, err)
+			result, err := exprFunc(nil, nil)
+			assert.NoError(t, err)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
+func Test_Hour_Error(t *testing.T) {
+	var getter ottl.TimeGetter[any] = &ottl.StandardTimeGetter[any]{
+		Getter: func(ctx context.Context, tCtx any) (any, error) {
+			return "not a time", nil
+		},
+	}
+	exprFunc, err := Hour(getter)
+	assert.NoError(t, err)
+	result, err := exprFunc(context.Background(), nil)
+	assert.Nil(t, result)
+	assert.Error(t, err)
+}

pkg/ottl/ottlfuncs/functions.go

@@ -40,6 +40,7 @@ func converters[K any]() []ottl.Factory[K] {
 		NewDurationFactory[K](),
 		NewExtractPatternsFactory[K](),
 		NewFnvFactory[K](),
+		NewHourFactory[K](),
 		NewHoursFactory[K](),
 		NewIntFactory[K](),
 		NewIsBoolFactory[K](),