Add documentation for API key security parameters and metrics

docs/config.md

@@ -3,7 +3,9 @@
 Configuration is handled by [Spring Boot](https://docs.spring.io/spring-boot/docs/current/reference/html/boot-features-external-config.html),
 which supports properties files, YAML files, environment variables and command-line arguments for setting config values.
 
-As a general rule, Prebid Cache will immediately fails on startup if any of required properties is missing or invalid.
+Both `_` and `-` can be used as a delimiters. As an example, both `api.cache_path` and `api.cache-path` refer to the same parameter.
+
+As a general rule, Prebid Cache will immediately fail on startup if any of required properties is missing or invalid.
 
 The next sections describes how to set up project configuration.
 
@@ -14,6 +16,8 @@ The next sections describes how to set up project configuration.
 - `api.cache_path` - set path for cache endpoint.
 - `api.module_storage_path` - set path for storage endpoint.
 - `api.api_key` - set API key.
+- `api.cache_write_secured` - if `true`, POST requests to `/cache` will require a valid API key.
+- `api.external_UUID_secured` - if `true`, providing external UUID will additionally require a valid API key.
 
 #### Cors
 - `cors.enabled` - toggle for cors.

docs/metrics.md

@@ -0,0 +1,21 @@
+# Metrics
+
+`{prefix}` can be `read` or `write` but doesn't automatically mean that metric will appear in both.
+
+- `pbc.{prefix}.request.duration` - the time it took to store or retrieve creative in ns.
+- `pbc.{prefix}.request` - the count of incoming requests. 
+- `pbc.{prefix}.trustedRequest` - the count of incoming requests carrying a matching API key.
+- `pbc.request.invalid` - the count of incoming requests using invalid endpoint or method.
+- `pbc.{prefix}.err.timedOut` - the count of timed out requests. Usually indicates an issue with the storage backend.
+- `pbc.{prefix}.err.missingId` - the count of requests that carried a UUID that was not present in storage backend.
+- `pbc.{prefix}.err.badRequest` - the count of malformed or otherwise invalid requests.
+- `pbc.{prefix}.err.unauthorized` - the count of unauthorized requests.
+- `pbc.{prefix}.err.unknown` - the count of unknown (not falling into the defined categories) errors.
+- `pbc.{prefix}.err.db` - the count of storage backend errors.
+- `pbc.{prefix}.json` - the count of JSON (banner) creatives.
+- `pbc.{prefix}.xml` - the count of XML (video) creatives.
+- `pbc.err.secondaryWrite` - the count of secondary write errors.
+- `pbc.err.existingId` - the count of errors due to existing UUID key in the storage backend. 
+- `pbc.err.rejectedExternalId` - the count of rejected writes due to specifying external UUID not being allowed.
+- `pbc.proxy.success` - the count of successful proxying requests.
+- `pbc.proxy.failure` - the count of failed proxying requests.