fromEnv detector throws an error when OTEL_RESOURCE_ATTRIBUTES is not set.

[seanschade]

Description

If you set OTEL_SERVICE_NAME, but do not set OTEL_RESOURCE_ATTRIBUTES an errMissingValue occurs.

"error": "partial resource: missing value: []"

Environment

• OS: Darwin
• Architecture: amd64
• Go Version: 1.16.3
• opentelemetry-go version: v1.0.0-RC1 and v1.0.0-RC2

Steps To Reproduce

Add the following test case to env_test.go, and it will fail.

func TestNoResourceAttributesSet(t *testing.T) {
	store, err := ottest.SetEnvVariables(map[string]string{
		svcNameKey:      "bar",
	})
	require.NoError(t, err)
	defer func() { require.NoError(t, store.Restore()) }()
	detector := &fromEnv{}
	res, err := detector.Detect(context.Background())
	require.NoError(t, err)
	assert.Equal(t, res, NewSchemaless(
		semconv.ServiceNameKey.String("bar"),
	))
}

constructOTResources is being called with an empty string. We should check the input, and return an empty resource if no attributes are provided.

func constructOTResources(s string) (*Resource, error) {
        // len(pairs) will equal 1 with an empty string
        if s == "" {
            return Empty(), nil
        }
	pairs := strings.Split(s, ",") // len(pairs) will equal 1 with an empty string
	attrs := []attribute.KeyValue{}
	var invalid []string
	for _, p := range pairs {
		field := strings.SplitN(p, "=", 2)
		if len(field) != 2 {
			invalid = append(invalid, p)
			continue
		}
		k, v := strings.TrimSpace(field[0]), strings.TrimSpace(field[1])
		attrs = append(attrs, attribute.String(k, v))
	}
	var err error
	if len(invalid) > 0 {
		err = fmt.Errorf("%w: %v", errMissingValue, invalid)
	}
	return NewSchemaless(attrs...), err
}

Expected behavior

No error should occur.

[seanschade]

Pull request to fix the bug.

#2139