fix(collection): set custom content type for cloudwatch logs (#120)

CloudWatch logs do not have an inferrable file type. This commit
explicitly overrides the content type based on the file name, since we
know where the logs will show up. For now, us a custom content type to
identify cloudwatch logs. This will allow us to handle this data in a
more targeted manner within filedrop.

apps/collection/template.yaml

@@ -200,7 +200,10 @@ Resources:
                   - !Join [",", !Ref SourceTopicArns]
         ContentTypeOverrides: !Join
             - ","
-            - !Ref ContentTypeOverrides
+            - - !Sub "s3://${Bucket}/cloudwatchlogs/=application/x-aws-cloudwatchlogs"
+              - !Join
+                - ","
+                - !Ref ContentTypeOverrides
         NameOverride: !If
           - UseStackName
           - !Ref 'AWS::StackName'

handler/forwarder/overrides.go

@@ -42,6 +42,13 @@ func (c *contentTypeOverride) Match(s string) string {
 func NewContentTypeOverrides(kvs []string, delimiter string) (Matcher, error) {
 	var m matches
 	for _, pair := range kvs {
+		if pair == "" {
+			// treat empty values as noops
+			// this overcomes a cloudformation limitation in concatenating
+			// multiple values, which can result in trailing commas
+			continue
+		}
+
 		split := strings.SplitN(pair, delimiter, 2)
 		if len(split) != 2 {
 			return nil, fmt.Errorf("error parsing %q: %w", pair, errMissingDelimiter)

handler/forwarder/overrides_test.go

@@ -13,7 +13,7 @@ func TestContentTypeOverridesErrors(t *testing.T) {
 	t.Parallel()
 
 	testcases := []string{
-		"",
+		"nonono",
 		"\\=",
 	}
 
@@ -40,6 +40,7 @@ func TestContentTypeOverrides(t *testing.T) {
 		{
 			ContentTypeOverrides: []string{
 				".*=application/json",
+				"",
 			},
 			Delimiter: "=",
 			Expect: map[string]string{