[v17] Update billing metrics doc with MWI information

docs/pages/usage-billing.mdx

@@ -130,6 +130,31 @@ hourly average. For example, if you were to create then delete 10 servers three
 times in one hour, Teleport would display 10 servers at any given time. However,
 for the entire hour, Teleport would report 30 protected servers.
 
+### Machine and Workload Identities
+
+The Machine and Workload Identity (MWI) metric is an aggregate of
+* [Bots](reference/architecture/machine-id-architecture.mdx#what-is-a-bot)
+* [Bot Instances](reference/architecture/machine-id-architecture.mdx#what-is-a-bot)
+* [SPIFFE IDs](enroll-resources/workload-identity/spiffe.mdx#spiffe-ids-and-trust-domains)
+
+We aggregate Bots, Bot Instances and unique SPIFFE IDs during each day on an hourly basis, and take
+an hourly average to compute a daily Bot, Bot Instance and SPIFFE ID count. Then we average the daily
+counts over a monthly period, which starts on the subscription start date and ends on each monthly
+anniversary thereafter.
+
+If you recreate a single Bot or Bot Instance more than once an hour, this will affect the
+hourly average. For example, if you were to create then delete 10 Bots three
+times in one hour, Teleport would display 10 Bots at any given time. However,
+for the entire hour, Teleport would report 30 Bots.
+
+For SPIFFE IDs, unique means that we only count a SPIFFE ID once during the calculation period,
+even if it is used for multiple services. For example, if ten services running 24 hours per day
+share the ID `spiffe://mydomain.cloud.gravitational.io/web-service`, Teleport would report that 
+as one SPIFFE ID for billing.
+
+The sum of your calculated Bots, Bot Instances, and unique SPIFFE IDs is your
+total MWI for the billing period.
+
 ## Usage measurement for billing
 
 We aggregate all counts of the billing metrics on a monthly basis starting on