allow custom tags on transactions (#699)

currently the only tags added to transactions are on the scope at the
time of finish, but sometimes it is useful to have tags that aren't
for the scope but just for the transaction

sentry-core/src/performance.rs

@@ -260,6 +260,14 @@ impl TransactionOrSpan {
         }
     }
 
+    /// Sets a tag to a specific value.
+    pub fn set_tag<V: ToString>(&self, key: &str, value: V) {
+        match self {
+            TransactionOrSpan::Transaction(transaction) => transaction.set_tag(key, value),
+            TransactionOrSpan::Span(span) => span.set_tag(key, value),
+        }
+    }
+
     /// Get the TransactionContext of the Transaction/Span.
     ///
     /// Note that this clones the underlying value.
@@ -492,6 +500,14 @@ impl Transaction {
         }
     }
 
+    /// Sets a tag to a specific value.
+    pub fn set_tag<V: ToString>(&self, key: &str, value: V) {
+        let mut inner = self.inner.lock().unwrap();
+        if let Some(transaction) = inner.transaction.as_mut() {
+            transaction.tags.insert(key.into(), value.to_string());
+        }
+    }
+
     /// Returns an iterating accessor to the transaction's
     /// [`extra` field](protocol::Transaction::extra).
     ///
@@ -645,6 +661,12 @@ impl Span {
         span.data.insert(key.into(), value);
     }
 
+    /// Sets a tag to a specific value.
+    pub fn set_tag<V: ToString>(&self, key: &str, value: V) {
+        let mut span = self.span.lock().unwrap();
+        span.tags.insert(key.into(), value.to_string());
+    }
+
     /// Returns a smart pointer to the span's [`data` field](protocol::Span::data).
     ///
     /// Since [`Data`] implements `Deref` and `DerefMut`, this can be used to read and mutate