Docs: Documentation for Prebid Cache traceability improvements (prebid#3774)

Author: Net-burst

docs/config-app.md

@@ -285,6 +285,7 @@ For `JVM` metrics
 - `cache.path` - set the external Cache Service path, for example `/cache`.
 - `storage.pbc.enabled` - If set to true, this will allow storing modules’ data in third-party storage.
 - `storage.pbc.path` - set the external Cache Service path for module caching, for example `/pbc-storage`.
+- `cache.api-key-secured` - if set to `true`, will cause Prebid Server to add a special API key header to Prebid Cache requests.
 - `pbc.api.key` - set the external Cache Service api key for secured calls.
 - `cache.query` - appends to the cache path as query string params (used for legacy Auction requests).
 - `cache.banner-ttl-seconds` - how long (in seconds) banner will be available via the external Cache Service.
@@ -294,6 +295,7 @@ for particular publisher account. Overrides `cache.banner-ttl-seconds` property.
 - `cache.account.<ACCOUNT>.video-ttl-seconds` - how long (in seconds) video creative will be available in Cache Service 
 for particular publisher account. Overrides `cache.video-ttl-seconds` property.
 - `cache.default-ttl-seconds.{banner, video, audio, native}` - a default value how long (in seconds) a creative of the specific type will be available in Cache Service
+- `cache.append-trace-info-to-cache-id` - if set to `true`, causes the addition account ID and datacenter to cache UUID: _ACCOUNT-DATACENTER-remainderOfUUID_. Implies that cache UUID will be generated by the Prebid Server. 
 
 ## Application settings (account configuration, stored ad unit configurations, stored requests)
 Preconfigured application settings can be obtained from multiple data sources consequently: 

docs/metrics.md

@@ -104,11 +104,13 @@ Following metrics are collected and submitted if account is configured with `det
 - `prebid_cache.requests.ok` - timer tracking how long did successful cache requests take
 - `prebid_cache.requests.err` - timer tracking how long did failed cache requests take
 - `prebid_cache.creative_size.<creative_type>` - histogram tracking creative sizes for specific type
+- `prebid_cache.creative_ttl.<creative_type>` - histogram tracking creative TTL for specific type
 
 ## Prebid Cache per-account metrics
 - `account.<account-id>.prebid_cache.requests.ok` - timer tracking how long did successful cache requests take when incoming request was from `<account-id>`
 - `account.<account-id>.prebid_cache.requests.err` - timer tracking how long did failed cache requests take when incoming request was from `<account-id>`
 - `account.<account-id>.prebid_cache.creative_size.<creative_type>` - histogram tracking creative sizes for specific type when incoming request was from `<account-id>`
+- `account.<account-id>.prebid_cache.creative_ttl.<creative_type>` - histogram tracking creative TTL for specific type when incoming request was from `<account-id>`
 
 ## /cookie_sync endpoint metrics
 - `cookie_sync_requests` - number of requests received