feat: define tags for all observations (#275)

See ooni/probe#2483 and ooni/probe-cli#1142.

data-formats/df-001-httpt.md

@@ -41,6 +41,7 @@ probes always set this field to `null`.
     "response_length": 1234,
     "t0": 0.9,
     "t": 1.0,
+    "tags": [],
     "transaction_id": 1
 }
 ```
@@ -77,6 +78,9 @@ the amount of time spent performing the operation);
 measured in the moment in which `failure` is determined (`t - t0` gives you
 the amount of time spent performing the operation);
 
+- `tags` (`[]string`; optional): list of tags for this event. This is useful to
+understand what part of a complex measurement generated an event.
+
 - `transaction_id` (`int`; optional; since 2020-01-11): the set of operations
 to which this event belongs to (typically an HTTP transaction or a DNS
 round trip). A zero or missing value means we don't know the transaction

data-formats/df-002-dnst.md

@@ -34,6 +34,7 @@ code. See this directory's [README](README.md) for the basic concepts.
     "resolver_address": "8.8.8.8:53",
     "t0": 1.007,
     "t": 1.114,
+    "tags": [],
     "transaction_id": 1,
 
     // specified but unused or deprecated fields
@@ -95,6 +96,9 @@ the amount of time spent performing the operation);
 measured in the moment in which `failure` is determined (`t - t0` gives you
 the amount of time spent performing the operation);
 
+- `tags` (`[]string`; optional): list of tags for this event. This is useful to
+understand what part of a complex measurement generated an event.
+
 - `transaction_id` (`int`; optional; since 2020-01-11): the set of operations
 to which this event belongs to (typically an HTTP transaction or a DNS
 round trip). A zero or missing value means we don't know the transaction

data-formats/df-005-tcpconnect.md

@@ -29,6 +29,7 @@ basic concepts.
     "status": {},
     "t0": 1.011,
     "t": 1.114,
+    "tags": [],
     "transaction_id": 1,
 
     // deprecated or unused fields
@@ -58,6 +59,9 @@ the amount of time spent performing the operation);
 measured in the moment in which `failure` is determined (`t - t0` gives you
 the amount of time spent performing the operation);
 
+- `tags` (`[]string`; optional): list of tags for this event. This is useful to
+understand what part of a complex measurement generated an event.
+
 - `transaction_id` (`int`; optional; since 2020-01-11): the set of operations
 to which this event belongs to (typically an HTTP transaction or a DNS
 round trip). A zero or missing value means we don't know the transaction