Retrieve FRED economic time series observations with transformations for a set of indicators

Steps

1. Register for a free API key at fred.stlouisfed.org/docs/api/api_key.html
2. Look up series IDs by searching the FRED website or calling GET https://api.stlouisfed.org/fred/series/search?search_text={term}&api_key={key}&file_type=json
3. Fetch observations by calling GET https://api.stlouisfed.org/fred/series/observations with params: series_id, api_key, file_type=json, observation_start, observation_end, units (lin, chg, ch1, pch, pc1, pca, cch, cca, log), and frequency (d, w, bw, m, q, sa, a)
4. For vintage/revision history use the realtime_start and realtime_end params on the same observations endpoint to retrieve data as it was published on a specific date
5. To retrieve release metadata, call GET https://api.stlouisfed.org/fred/series/release?series_id={id}&api_key={key}&file_type=json
6. Batch multiple series into a single request by calling GET https://api.stlouisfed.org/fred/release/series?release_id={id} to get all series in a release, then loop observations calls